Develop Reference

I want my drop down also work as a link in bootstrap

While I am clicking this dropdown menu it's not going to the product section it's only expanding the menu and showing me Single-phase and three-phase but not also going to the product section that I made. Currently, I am using Bootstrap 5
Here is the code:(for navbar)
<li class="nav-item dropdown a-group">
<a class="nav-link dropdown-toggle" id="navbarDropdown" role="button"
data-bs-toggle="dropdown" aria-expanded="false" href="#product">
Products
</a>
<ul class="dropdown-menu" aria-labelledby="navbarDropdown">
<li><a class="nav-link dropdown-item" href="#">Single Phase</a></li>
<li>
<hr class="dropdown-divider">
</li>
<li><a class="nav-link dropdown-item" href="#">Three Phase</a></li>
</ul>
</li>
Here is product section:
<section id="product">
<div class="container">
<header class="section-header">
<h3><b>Check out our products!</b></h3>
</header>
<div class="carousel">
<div id="carouselExampleInterval" class="carousel slide" data-bs-ride="carousel">
<div class="carousel-inner">
<div class="carousel-item active justify-content-center" data-bs-interval="10000">
<img src="image/intro-one.png" height="500px" class="d-block" alt="...">
<div class="carousel-caption d-none d-md-block">
<h5>First slide label</h5>
<p>Some representative placeholder content for the first slide.</p>
</div>
</div>
<div class="carousel-item justify-content-center" data-bs-interval="2000">
<img src="image/intro-two.png" height="500px" class="d-block " alt="...">
<div class="carousel-caption d-none d-md-block">
<h5>First slide label</h5>
<p>Some representative placeholder content for the first slide.</p>
</div>
</div>
<div class="carousel-item justify-content-center">
<img src="image/intro-one.png" height="500px" class="d-block" alt="...">
<div class="carousel-caption d-none d-md-block">
<h5>First slide label</h5>
<p>Some representative placeholder content for the first slide.</p>
</div>
</div>
</div>
<button class="carousel-control-prev" type="button" data-bs-target="#carouselExampleInterval"
data-bs-slide="prev">
<span class="carousel-control-prev-icon" aria-hidden="true"></span>
<span class="visually-hidden">Previous</span>
</button>
<button class="carousel-control-next" type="button" data-bs-target="#carouselExampleInterval"
data-bs-slide="next">
<span class="carousel-control-next-icon" aria-hidden="true"></span>
<span class="visually-hidden">Next</span>
</button>
</div>
</div>
</div>
</section>

Just remove data-bs-toggle="dropdown" and add style for hover
.dropdown:hover > .dropdown-menu { display: block; }
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/bootstrap#4.6.0/dist/js/bootstrap.bundle.min.js"></script>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap#4.6.0/dist/css/bootstrap.min.css" integrity="sha384-B0vP5xmATw1+K9KRQjQERJvTumQW0nPEzvF6L/Z6nronJ3oUOFUFpCjEUQouq2+l" crossorigin="anonymous">
<nav class="navbar navbar-expand-lg navbar-light bg-light">
<a class="navbar-brand" href="#">Navbar</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="navbar-nav mr-auto">
<li class="nav-item">
<a class="nav-link" href="#">Link</a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="http://google.com" id="navbarDropdown" role="button" aria-haspopup="true" aria-expanded="false">
Dropdown
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="#">Something else here</a>
</div>
</li>
</ul>
</div>
</nav>

Adding basket icon quantity of products in the basket in CSS

I am a beginner web developer. I use in my project Bootstrap 4.
I have this code:
<nav class="navbar navbar-expand-lg text-body px-md-0 pb-md-0 navbar-light w-100">
<a class="navbar-brand" href="#">Logo</a>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarmenu"
aria-controls="navbarmenu" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon "></span>
</button>
<div class="collapse navbar-collapse" id="navbarmenu">
<ul class="navbar-nav">
<form class="mx-lg-5">
<div class="inner-addon rounded-0 navbar-search-form">
<i class="fa fa-search"></i>
<input type="text" class="form-control" placeholder="Szukaj produktu"
aria-label="Szukaj produktu"/>
</div>
</form>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="dropdown1" data-toggle="dropdown"
aria-haspopup="true"
aria-expanded="false">Option <i class="fas fa-angle-down"></i></a>
<div class="dropdown-menu" aria-labelledby="dropdown01">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Something else here</a>
</div>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="#" id="dropdown2" data-toggle="dropdown"
aria-haspopup="true"
aria-expanded="false">Option <i class="fas fa-angle-down"></i></a>
<div class="dropdown-menu" aria-labelledby="dropdown01">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Something else here</a>
</div>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Kontakt</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#"><img src="img/profile.jpg" class="pl-lg-4 pr-2"><span class="d-lg-none d-xl-inline">Zaloguj się</span></a>
</li>
<li class="nav-item">
<a class="nav-link" href="#"><img src="img/basket.jpg" class="pl-lg-4 pr-2"><span class="d-lg-none d-xl-inline">Koszyk</span></a>
</li>
</ul>
</div>
</nav>
</div>
</div>
Preview: http://serwer1356363.home.pl/_nauka/
I need to add the number of products in the basket to my basket icon as this screen:
https://ibb.co/PcvM9Dr
How can I make it?
Please help me

You can use this code
Additional CSS :
.count{
width: 15px;
height: 15px;
border-radius: 15px;
display: -webkit-flex;
display: flex;
-webkit-align-items: center;
align-items: center;
-webkit-justify-content: center;
justify-content: center;
position: absolute;
top: 15px;
left: 60px;
background: #ff420f;
}
Finally in HTML:
<li class="nav-item position-relative">
<a class="nav-link" href="#"><img src="img/basket.jpg" class="pl-lg-4 pr-2">
<span class="badge badge-primary count">5</span >
<span class="ml-2 d-lg-none d-xl-inline">Koszyk</span></a>
</li>
Dont forget to add class .position-relative inside li.nav-item

Bootstrap 4 Center Nav Dropdown

I am using bootstrap 4 to create a navigational menu. My navigational items each contain a dropdown menu. When clicked I want this menu to display the same size and be horizontally centered on the page. Ideally, I want this dropdown menu to be the same size as the ul element. At the moment I can't figure out how to do this as I am relatively new to bootstrap.
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css">
<nav class="navbar navbar-expand-md navbar-light bg-primary py-0">
<div class="container h-100">
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse h-100" id="navbarSupportedContent">
<ul class="navbar-nav mr-auto mx-auto h-100">
<li class="nav-item dropdown px-2">
<a class="nav-link dropdown-toggle text-white" href="/category/flat-roof-materials" id="flatRoofingDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Flat Roofing
</a>
<div class="dropdown-menu border border-primary row mx-auto" aria-labelledby="flatRoofingDropdown">
<div class="container">
<div class="row">
<div class="col-md-4">
<a class="dropdown-item text-primary" href="/category/epdm-rubber-roofing">EPDM Rubber Roofing</a>
<a class="dropdown-item" href="/category/classicbond">ClassicBond</a>
<a class="dropdown-item" href="/category/firestone-rubbercover">Firestone</a>
<a class="dropdown-item" href="/category/duoply">Duoply</a>
<a class="dropdown-item text-primary" href="/category/fibreglass-grp-roofing-systems">Fibreglass GRP</a>
<a class="dropdown-item" href="/category/fibreglass-grp-roofing-systems?filter232=Cromar&page=1">ProGRP</a>
<a class="dropdown-item" href="/category/flexitec-2020">Flexitec 2020</a>
<a class="dropdown-item" href="/tools/flexitec-calculator">Flexitec 2020 Roof Calculator</a>
<a class="dropdown-item" href="/tools/grp1010-calculator">GRP Roof 1010</a>
<a class="dropdown-item" href="/category/ultraflex">Ultraflex</a>
<a class="dropdown-item text-primary" href="/category/torch-on-felt">Roofing Felt</a>
<a class="dropdown-item" href="/category/torch-on-felt">Torch on felts</a>
<a class="dropdown-item text-primary" href="/category/timber-sheeting">Timber Sheeting</a>
<a class="dropdown-item" href="/category/osb">OSB</a>
</div>
<div class="col-md-4">
<a class="dropdown-item text-primary" href="/category/green-roofing">Green Roof System</a>
<a class="dropdown-item" href="/category/green-roofing?filter232=GrufeKit&page=1">Grufe kit</a>
<a class="dropdown-item text-primary" href="/category/liquid-waterproofing-systems">Liquid Waterproofing</a>
<a class="dropdown-item" href="/category/elastathane">Elastathane 25</a>
<a class="dropdown-item" href="/category/hydrosil-silicone-roof-coating">Hydrosil</a>
<a class="dropdown-item" href="/tools/flexitec-calculator">Liquid Rubber</a>
<a class="dropdown-item text-primary" href="#">Skylights</a>
<a class="dropdown-item" href="#">Flat Glass Rooflights</a>
<a class="dropdown-item" href="#">Domes</a>
<a class="dropdown-item text-primary" href="#">Flat Roof Outlets</a>
<a class="dropdown-item" href="/category/flat-roof-outlets">Water Outlets</a>
<a class="dropdown-item" href="/category/flat-roof-vents">Flat Roof Vents</a>
</div>
<div class="col-md-4">
<a class="dropdown-item navbar-brand text-primary h5" href="/category/epdm-rubber-roofing">EPDM Rubber Roofing</a>
<a class="dropdown-item" href="/category/classicbond">ClassicBond</a>
<a class="dropdown-item" href="/category/firestone-rubbercover">Firestone</a>
<a class="dropdown-item" href="/category/duoply">Duoply</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item navbar-brand text-primary" href="/category/fibreglass-grp-roofing-systems">Fibreglass GRP</a>
<a class="dropdown-item" href="/category/fibreglass-grp-roofing-systems?filter232=Cromar&page=1">ProGRP</a>
<a class="dropdown-item" href="/category/flexitec-2020">Flexitec 2020</a>
<a class="dropdown-item" href="/tools/flexitec-calculator">Flexitec 2020 Roof Calculator</a>
<a class="dropdown-item" href="/tools/grp1010-calculator">GRP Roof 1010</a>
<a class="dropdown-item" href="/category/ultraflex">Ultraflex</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item navbar-brand text-primary" href="/category/torch-on-felt">Roofing Felt</a>
<a class="dropdown-item" href="/category/torch-on-felt">Torch on felts</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item navbar-brand text-primary" href="/category/timber-sheeting">Timber Sheeting</a>
<a class="dropdown-item" href="/category/osb">OSB</a>
</div>
</div>
</div>
</div>
</li>
</ul>
</div>
</div>
</nav>
<script src="https://code.jquery.com/jquery-3.4.1.slim.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/popper.js#1.16.0/dist/umd/popper.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/js/bootstrap.min.js"></script>
Current behaviour:
Expected outcome, centered, and the same size:
result from suggested fix:

Add below CSS.
.dropdown {
position: initial;
}
#media (min-width: 768px) {
.dropdown-menu {
width: 50rem; /* JUST FOR STYLING PURPOSE OF DROPDOWN */
position: absolute;
left: 50% !important;
transform: translateX(-50%) !important;
}
}
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css">
<nav class="navbar navbar-expand-md navbar-light bg-primary py-0">
<div class="container h-100">
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse h-100" id="navbarSupportedContent">
<ul class="navbar-nav mr-auto mx-auto h-100">
<li class="nav-item dropdown px-2">
<a class="nav-link dropdown-toggle text-white" href="/category/flat-roof-materials" id="flatRoofingDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Flat Roofing
</a>
<div class="dropdown-menu border border-primary row mx-auto" aria-labelledby="flatRoofingDropdown">
<div class="container">
<div class="row">
<div class="col-md-4">
<a class="dropdown-item text-primary" href="/category/epdm-rubber-roofing">EPDM Rubber Roofing</a>
<a class="dropdown-item" href="/category/classicbond">ClassicBond</a>
<a class="dropdown-item" href="/category/firestone-rubbercover">Firestone</a>
<a class="dropdown-item" href="/category/duoply">Duoply</a>
<a class="dropdown-item text-primary" href="/category/fibreglass-grp-roofing-systems">Fibreglass GRP</a>
<a class="dropdown-item" href="/category/fibreglass-grp-roofing-systems?filter232=Cromar&page=1">ProGRP</a>
<a class="dropdown-item" href="/category/flexitec-2020">Flexitec 2020</a>
<a class="dropdown-item" href="/tools/flexitec-calculator">Flexitec 2020 Roof Calculator</a>
<a class="dropdown-item" href="/tools/grp1010-calculator">GRP Roof 1010</a>
<a class="dropdown-item" href="/category/ultraflex">Ultraflex</a>
<a class="dropdown-item text-primary" href="/category/torch-on-felt">Roofing Felt</a>
<a class="dropdown-item" href="/category/torch-on-felt">Torch on felts</a>
<a class="dropdown-item text-primary" href="/category/timber-sheeting">Timber Sheeting</a>
<a class="dropdown-item" href="/category/osb">OSB</a>
</div>
<div class="col-md-4">
<a class="dropdown-item text-primary" href="/category/green-roofing">Green Roof System</a>
<a class="dropdown-item" href="/category/green-roofing?filter232=GrufeKit&page=1">Grufe kit</a>
<a class="dropdown-item text-primary" href="/category/liquid-waterproofing-systems">Liquid Waterproofing</a>
<a class="dropdown-item" href="/category/elastathane">Elastathane 25</a>
<a class="dropdown-item" href="/category/hydrosil-silicone-roof-coating">Hydrosil</a>
<a class="dropdown-item" href="/tools/flexitec-calculator">Liquid Rubber</a>
<a class="dropdown-item text-primary" href="#">Skylights</a>
<a class="dropdown-item" href="#">Flat Glass Rooflights</a>
<a class="dropdown-item" href="#">Domes</a>
<a class="dropdown-item text-primary" href="#">Flat Roof Outlets</a>
<a class="dropdown-item" href="/category/flat-roof-outlets">Water Outlets</a>
<a class="dropdown-item" href="/category/flat-roof-vents">Flat Roof Vents</a>
</div>
<div class="col-md-4">
<a class="dropdown-item navbar-brand text-primary h5" href="/category/epdm-rubber-roofing">EPDM Rubber Roofing</a>
<a class="dropdown-item" href="/category/classicbond">ClassicBond</a>
<a class="dropdown-item" href="/category/firestone-rubbercover">Firestone</a>
<a class="dropdown-item" href="/category/duoply">Duoply</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item navbar-brand text-primary" href="/category/fibreglass-grp-roofing-systems">Fibreglass GRP</a>
<a class="dropdown-item" href="/category/fibreglass-grp-roofing-systems?filter232=Cromar&page=1">ProGRP</a>
<a class="dropdown-item" href="/category/flexitec-2020">Flexitec 2020</a>
<a class="dropdown-item" href="/tools/flexitec-calculator">Flexitec 2020 Roof Calculator</a>
<a class="dropdown-item" href="/tools/grp1010-calculator">GRP Roof 1010</a>
<a class="dropdown-item" href="/category/ultraflex">Ultraflex</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item navbar-brand text-primary" href="/category/torch-on-felt">Roofing Felt</a>
<a class="dropdown-item" href="/category/torch-on-felt">Torch on felts</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item navbar-brand text-primary" href="/category/timber-sheeting">Timber Sheeting</a>
<a class="dropdown-item" href="/category/osb">OSB</a>
</div>
</div>
</div>
</div>
</li>
</ul>
</div>
</div>
</nav>
<script src="https://code.jquery.com/jquery-3.4.1.slim.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/popper.js#1.16.0/dist/umd/popper.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/js/bootstrap.min.js"></script>

How to get Javascript to change styles?

My JavaScript is not changing the attributes upon calling them from the "Change your style" button. It should change all four paragraphs under the images to a different style. Any assistance would be appreciated. I feel as if it's just a typo somewhere.
Here is my code:
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-
to-fit=no">
<title>Week4-1.html</title>
<link rel="stylesheet"
href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css"
integrity="sha384-
Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm"
crossorigin="anonymous">
<script
src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"
integrity="sha384-
JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl"
crossorigin="anonymous"></script>
<script src="../js/week4-1.js"></script>
<link rel="stylesheet" href="../css/homework.css">
</head>
<body>
<script src="../js/week4-1.js"></script>
<!--Navigation to different links and parts of the website-->
<nav class="navbar navbar-expand-md navbar-dark fixed-top bg-dark">
<a class="navbar-brand" href="#">My Rummage Store</a>
<button class="navbar-toggler" type="button" data-toggle="collapse"
data-target="#navbarsExampleDefault" aria-
controls="navbarsExampleDefault" aria-expanded="false" aria-label="Toggle
navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarsExampleDefault">
<ul class="navbar-nav mr-auto">
<li class="nav-item active">
<a class="nav-link" href="#">Home<span class="sr-only">(current)</span></a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="http://example.com" id="dropdown01" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Products</a>
<div class="dropdown-menu" aria-labelledby="dropdown01">
<a class="dropdown-item" href="#">Rummage Items</a>
<a class="dropdown-item" href="#">Unique Items</a>
<a class="dropdown-item" href="#">Love that Ink Pen Company</a>
</div>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="http://example.com" id="dropdown02" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Who we are</a>
<div class="dropdown-menu" aria-labelledby="dropdown01">
<a class="dropdown-item" href="#">About Us</a>
<a class="dropdown-item" href="#">Our Vision</a>
<a class="dropdown-item" href="#">Contact Us</a>
</div>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="http://example.com" id="dropdown03" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Links</a>
<div class="dropdown-menu" aria-labelledby="dropdown01">
<a class="dropdown-item" href="../pages/home.html">Home</a>
<a class="dropdown-item" href="../pages/week1-1.html">Week 1-1</a>
<a class="dropdown-item" href="../pages/week1-2.html">Week 1-2</a>
<a class="dropdown-item" href="../pages/week2-1.html">Week 2-1</a>
<a class="dropdown-item" href="../pages/week2-2.html">Week 2-2</a>
<a class="dropdown-item" href="../pages/week3-1.html">Week 3-1</a>
<a class="dropdown-item" href="../pages/week3-2.html">Week 3-2</a>
<a class="dropdown-item" href="../pages/week4-1.html">Week 4-1</a>
<a class="dropdown-item" href="../pages/week4-2.html">Week 4-2</a>
<a class="dropdown-item" href="../pages/week5-1.html">Week 5-1</a>
<a class="dropdown-item" href="../pages/week5-2.html">Week 5-2</a>
<a class="dropdown-item" href="../pages/week6-1.html">Week 6-1</a>
<a class="dropdown-item" href="../pages/week6-2.html">Week 6-2</a>
<a class="dropdown-item" href="../pages/week7-1.html">Week 7-1</a>
<a class="dropdown-item" href="../pages/week7-2.html">Week 7-2</a>
<a class="dropdown-item" href="../pages/week8-1.html">Week 8-1</a>
<a class="dropdown-item" href="../pages/week8-2.html">Week 8-2</a>
</div>
</li>
</ul>
<form class="form-inline my-2 my-lg-0">
<input class="form-control mr-sm-2" type="text" placeholder="Search" aria-label="Search">
<button class="btn btn-outline-success my-2 my-sm-0" type="submit">Submit</button>
</form>
</div>
</nav>
<main role="main">
<!--Main jumbotron-->
<div class="jumbotron">
<div class="container">
<h1 class="display-3" style="text-align:center">Rummage Items</h1>
</div>
<!--Card to display a logo-->
<div class="card" style="width: 18rem; color:black">
<img class="card-img-top" src="../images/week2-1logo.jpg" alt="Card image cap">
<div class="card-body">
<h5 class="card-title"></h5>
<p class="card-text">Rummage it</p>
</div>
<!--Button to change paragraph font-->
<p>
<button type="button" onclick="changeStyles();" class="btn btn-primary btn-lg">Change the Style</button>
</p>
</div>
</div>
<!--Improved columns--->
<div class="container-fluid">
<button type="button" onclick="changeStyles();" class="btn btn-primary btn-lg">Change the Paragraph Style</button>
<div class="row">
<div class="col-md-3 home_box_border home_box_corner home_box_shadow home_box_gradient" div style="text-align:center; vertical-align:middle">
<hr />
<h2>Printers</h2>
<img class="img-circle1" src="../images/product4.jpg" alt="print" align="middle">
<p class="week4-1_Printers">The HPSR printer has a direct feed for over 30 different types of paper. Allowing for ultimate utilization. It also showcases the new style of modern printers. Multifunction printers allow for use of communication via fax.</p>
<p><a class="btn btn-secondary" href="#" role="button">Buy for $299.99 »</a></p>
</div>
<div class="col-md-3 home_box_border home_box_corner home_box_shadow home_box_gradient" div style="text-align:center; vertical-align:middle">
<hr />
<h2>Boots</h2>
<img class="img-circle1" src="../images/product3.jpg" alt="boots">
<p class="week4-1_Boots">The new rummage boot collection. Hand crafted leather for the ultimate rummaging experience. Your feet will thank you during your next rummaging expedition! We look forward to helping you try them on. </p>
<p><a class="btn btn-secondary" href="#" role="button">Buy for $69.99 »</a></p>
</div>
<div class="col-md-3 home_box_border home_box_corner home_box_gradient home_box_shadow" div style="text-align:center; vertical-align:middle">
<hr />
<h2>Lamps</h2>
<img class="img-circle1" src="../images/product5.jpg" alt="Lamp">
<p class="week4-1_Lamps">Lamps are a neccesity during night time. This lamp is hand crafted and made from the finest craftsman we could find. You will not regret buying this lamp, adding a nice piece of decor and bringing some brigtness in your life. </p>
<p><a class="btn btn-secondary" href="../pages/week1-2.html" role="button"> Buy for $39.99 »</a></p>
</div>
<div class="col-md-3 home_box_border home_box_corner home_box_gradient home_box_shadow" div style="text-align:center; vertical-align:middle">
<hr />
<h2>Dressers</h2>
<img class="img-circle1" src="../images/product2.jpg" alt="Lamp">
<p class="week4-1_Dressers">Our Dressers are one of a kind. We have so many variations that you can rummage through and find. Purchase on our site or come in today and take a look! Very sturdy and made with some of the best wood you can buy. </p>
<p><a class="btn btn-secondary" href="../pages/week1-2.html" role="button"> Buy for $239.99 »</a></p>
</div>
</div>
</div>
</main>
<!-- Footer Information -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>
Here is my js:
function injectStyles(rule) {
var div = $("<div />", {
html: '­<style>' + rule + '</style>'
}).appendTo("body")
}
function changeStyles() {
// alert("Hi");
// injectStyles('p { color: red; }');
injectStyles('.week4-1_Printers { border: 10px solid black; font-weight:
500; color: darkblue; border-radius: 10px; }');
injectStyles('.week4-1_Boots { border:10px solid black; font-weight: 500;
color: darkblue; border-radius: 10px; }');
injectStyles('.week4-1_Lamps { border:10px solid black; font-weight: 500;
color: darkblue; border-radius: 10px; }');
injectStyles('.week4-1_Dressers { border:10px solid black; font-weight: 500;
color: darkblue; border-radius: 10px; }');
}

I will update the
function injectStyles(rule) {
$("head").append('<style>' + rule + '</style>');
}
to append style in the head.

I don't think that your issue is with the button, as the code is being executed.
Instead of writing div elements why not just change the style directly? I changed your injectStyles function to accept two arguments: the class name of the elements you want to change and a JavaScript object that contains all the style values you want to set. Then, I just loop through every element that matches the class name, then set the style for every node in the style definition.
(you need to go into full screen to see the snippet work)
function injectStyles(className, styles) {
var elements = document.getElementsByClassName(className);
// loop through every element
for(var i=0; i < elements.length; i++) {
// loop through every style
for(var style in styles) {
elements[i].style[style] = styles[style];
}
}
}
function changeStyles() {
injectStyles('week4-1_Printers', { "border": "10px solid black", "font-weight": "500", "color": "darkblue", "border-radius": "10px" } );
injectStyles('week4-1_Boots', { "border": "10px solid black", "font-weight": "500", "color": "darkblue", "border-radius": "10px" } );
// injectStyles('.week4-1_Lamps { border:10px solid black; font-weight: 500; color: darkblue; border-radius: 10px; }');
// injectStyles('.week4-1_Dressers { border:10px solid black; font-weight: 500; color: darkblue; border-radius: 10px; }');
}
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1, shrink-
to-fit=no">
<title>Week4-1.html</title>
<link rel="stylesheet"
href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css"
integrity="sha384-
Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm"
crossorigin="anonymous">
<script
src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"
integrity="sha384-
JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl"
crossorigin="anonymous"></script>
<script src="./script.js"></script>
<link rel="stylesheet" href="../css/homework.css">
</head>
<body>
<script src="../js/week4-1.js"></script>
<!--Navigation to different links and parts of the website-->
<nav class="navbar navbar-expand-md navbar-dark fixed-top bg-dark">
<a class="navbar-brand" href="#">My Rummage Store</a>
<button class="navbar-toggler" type="button" data-toggle="collapse"
data-target="#navbarsExampleDefault" aria-
controls="navbarsExampleDefault" aria-expanded="false" aria-label="Toggle
navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarsExampleDefault">
<ul class="navbar-nav mr-auto">
<li class="nav-item active">
<a class="nav-link" href="#">Home<span class="sr-only">(current)</span></a>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="http://example.com" id="dropdown01" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Products</a>
<div class="dropdown-menu" aria-labelledby="dropdown01">
<a class="dropdown-item" href="#">Rummage Items</a>
<a class="dropdown-item" href="#">Unique Items</a>
<a class="dropdown-item" href="#">Love that Ink Pen Company</a>
</div>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="http://example.com" id="dropdown02" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Who we are</a>
<div class="dropdown-menu" aria-labelledby="dropdown01">
<a class="dropdown-item" href="#">About Us</a>
<a class="dropdown-item" href="#">Our Vision</a>
<a class="dropdown-item" href="#">Contact Us</a>
</div>
</li>
<li class="nav-item dropdown">
<a class="nav-link dropdown-toggle" href="http://example.com" id="dropdown03" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Links</a>
<div class="dropdown-menu" aria-labelledby="dropdown01">
<a class="dropdown-item" href="../pages/home.html">Home</a>
<a class="dropdown-item" href="../pages/week1-1.html">Week 1-1</a>
<a class="dropdown-item" href="../pages/week1-2.html">Week 1-2</a>
<a class="dropdown-item" href="../pages/week2-1.html">Week 2-1</a>
<a class="dropdown-item" href="../pages/week2-2.html">Week 2-2</a>
<a class="dropdown-item" href="../pages/week3-1.html">Week 3-1</a>
<a class="dropdown-item" href="../pages/week3-2.html">Week 3-2</a>
<a class="dropdown-item" href="../pages/week4-1.html">Week 4-1</a>
<a class="dropdown-item" href="../pages/week4-2.html">Week 4-2</a>
<a class="dropdown-item" href="../pages/week5-1.html">Week 5-1</a>
<a class="dropdown-item" href="../pages/week5-2.html">Week 5-2</a>
<a class="dropdown-item" href="../pages/week6-1.html">Week 6-1</a>
<a class="dropdown-item" href="../pages/week6-2.html">Week 6-2</a>
<a class="dropdown-item" href="../pages/week7-1.html">Week 7-1</a>
<a class="dropdown-item" href="../pages/week7-2.html">Week 7-2</a>
<a class="dropdown-item" href="../pages/week8-1.html">Week 8-1</a>
<a class="dropdown-item" href="../pages/week8-2.html">Week 8-2</a>
</div>
</li>
</ul>
<form class="form-inline my-2 my-lg-0">
<input class="form-control mr-sm-2" type="text" placeholder="Search" aria-label="Search">
<button class="btn btn-outline-success my-2 my-sm-0" type="submit">Submit</button>
</form>
</div>
</nav>
<main role="main">
<!--Main jumbotron-->
<div class="jumbotron">
<div class="container">
<h1 class="display-3" style="text-align:center">Rummage Items</h1>
</div>
<!--Card to display a logo-->
<div class="card" style="width: 18rem; color:black">
<img class="card-img-top" src="../images/week2-1logo.jpg" alt="Card image cap">
<div class="card-body">
<h5 class="card-title"></h5>
<p class="card-text">Rummage it</p>
</div>
<!--Button to change paragraph font-->
<p>
<button type="button" onclick="changeStyles();" class="btn btn-primary btn-lg">Change the Style</button>
</p>
</div>
</div>
<!--Improved columns--->
<div class="container-fluid">
<button type="button" onclick="changeStyles();" class="btn btn-primary btn-lg">Change the Paragraph Style</button>
<div class="row">
<div class="col-md-3 home_box_border home_box_corner home_box_shadow home_box_gradient" div style="text-align:center; vertical-align:middle">
<hr />
<h2>Printers</h2>
<img class="img-circle1" src="../images/product4.jpg" alt="print" align="middle">
<p class="week4-1_Printers">The HPSR printer has a direct feed for over 30 different types of paper. Allowing for ultimate utilization. It also showcases the new style of modern printers. Multifunction printers allow for use of communication via fax.</p>
<p><a class="btn btn-secondary" href="#" role="button">Buy for $299.99 »</a></p>
</div>
<div class="col-md-3 home_box_border home_box_corner home_box_shadow home_box_gradient" div style="text-align:center; vertical-align:middle">
<hr />
<h2>Boots</h2>
<img class="img-circle1" src="../images/product3.jpg" alt="boots">
<p class="week4-1_Boots">The new rummage boot collection. Hand crafted leather for the ultimate rummaging experience. Your feet will thank you during your next rummaging expedition! We look forward to helping you try them on. </p>
<p><a class="btn btn-secondary" href="#" role="button">Buy for $69.99 »</a></p>
</div>
<div class="col-md-3 home_box_border home_box_corner home_box_gradient home_box_shadow" div style="text-align:center; vertical-align:middle">
<hr />
<h2>Lamps</h2>
<img class="img-circle1" src="../images/product5.jpg" alt="Lamp">
<p class="week4-1_Lamps">Lamps are a neccesity during night time. This lamp is hand crafted and made from the finest craftsman we could find. You will not regret buying this lamp, adding a nice piece of decor and bringing some brigtness in your life. </p>
<p><a class="btn btn-secondary" href="../pages/week1-2.html" role="button"> Buy for $39.99 »</a></p>
</div>
<div class="col-md-3 home_box_border home_box_corner home_box_gradient home_box_shadow" div style="text-align:center; vertical-align:middle">
<hr />
<h2>Dressers</h2>
<img class="img-circle1" src="../images/product2.jpg" alt="Lamp">
<p class="week4-1_Dressers">Our Dressers are one of a kind. We have so many variations that you can rummage through and find. Purchase on our site or come in today and take a look! Very sturdy and made with some of the best wood you can buy. </p>
<p><a class="btn btn-secondary" href="../pages/week1-2.html" role="button"> Buy for $239.99 »</a></p>
</div>
</div>
</div>
</main>
<!-- Footer Information -->
If you want to keep the styles as a string you could parse it into an array using styles.split('delimiter'), but defining it as a JavaScript object makes it super-easy to process.

Hover issue in navbar with Bootstrap 4

I have created a navbar with Bootstrap 4. When I click on a drop-down menu link, it opens and remains open — when I hover on another drop-down link, they overlap.
I want it such that a dropdown link will be closed if I hover on another dropdown link.
Here is the code of my navbar drop-down:
.dropdown:hover>.dropdown-menu {
display: block;
}
<!DOCTYPE html>
<html lang="en">
<head>
<title>
Welcome - Maqbool Solutions
</title>
<link rel="shortcut icon" href="../images/favion_Y13_5.ico" type="image/x-icon">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css" integrity="sha384-PsH8R72JQ3SOdhVi3uxftmaW6Vc51MKb0q5P2rRUpPvrszuE4W1povHYgTpBfshb" crossorigin="anonymous">
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.3/umd/popper.min.js" integrity="sha384-vFJXuSJphROIrBnz7yo7oB41mKfc8JzQZiCq4NCceLEaO4IHwicKwpJf9c9IpFgh" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/js/bootstrap.min.js" integrity="sha384-alpBpkh1PFOepccYVYDB4do5UnbKysX5WZXm3XxPqe5iKTfUKjNkCk9SaVuEZflJ" crossorigin="anonymous"></script>
<script src="../../js/function.js"></script>
<link rel="stylesheet" href="css/main.css">
<link rel="stylesheet" href="css/style.css">
<link rel="stylesheet" href="font/css/font-awesome.css">
</head>
<body>
<div class="container-fluid">
<!--main container-->
<!-- navbar-->
<nav class="navbar navbar-expand-lg navbar-custom fixed-top" id="navbar" style=" height: 80px;">
<div id="logo">
<a href="index.html">
<h3 class="logo_text">Maqbool Solutions</h3>
</a>
</div>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="navbar-nav ml-auto" id="ul">
<li class="nav-item " align="center">
<i class="fa fa-home " aria-hidden="true"></i>
<a class="nav-link" href="index.html">Home <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item dropdown" align="center">
<i class="fa fa-cubes" aria-hidden="true"></i>
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Products
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="products/ims/ims.html">Institute Managment Systems</a>
<a class="dropdown-item" href="products/pnms/pnms.html">Pension Managment Systems</a>
<a class="dropdown-item" href="products/pms/pms.html">Property Managment Systems</a>
<a class="dropdown-item" href="products/labms/labms.html">Laboratory Managment Systems</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="products/apps/apps.html">Apps</a>
</div>
</li>
<li class="nav-item dropdown" align="center">
<i class="fa fa-cogs" aria-hidden="true"></i>
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Services
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="services/skill_service.html">Training and Skills</a>
<a class="dropdown-item" href="services/indestrial_services.html">Industry Expertise</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="services/web_service.html">Web services</a>
<a class="dropdown-item" href="services/software_service.html">Software services</a>
<a class="dropdown-item" href="services/animation_service.html">Animation services</a>
<a class="dropdown-item" href="services/graphic_service.html">Graphic services</a>
<a class="dropdown-item" href="services/game_service.html">Game services</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="services/apps_service.html">App services</a>
</div>
</li>
<li class="nav-item dropdown" align="center">
<i class="fa fa-pencil-square-o" aria-hidden="true"></i>
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Training
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="training/soft.html">Software Designing & Development</a>
<a class="dropdown-item" href="training/design.html">Designing & Composing</a>
<a class="dropdown-item" href="training/web.html">Web Designing & Development</a>
<a class="dropdown-item" href="training/animation.html">2D&3D Animation & Development </a>
<a class="dropdown-item" href="training/game.html">Game Development </a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="training/app.html">App Development </a>
</div>
</li>
<li class="nav-item dropdown" align="center">
<i class="fa fa-briefcase" aria-hidden="true"></i>
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
About Us
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="aboutus/hwr.html">Who we are</a>
<a class="dropdown-item" href="aboutus/our_vision.html">Our vision</a>
<a class="dropdown-item" href="aboutus/our_team.html">Our Team</a>
<a class="dropdown-item" href="aboutus/policy.html">Privacy & Policy</a>
</div>
</li>
<li class="nav-item dropdown" align="center">
<i class="fa fa-graduation-cap" aria-hidden="true"></i>
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Career
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="career/jobs.html">Jobs</a>
<a class="dropdown-item" href="career/inter.html">Internship</a>
<a class="dropdown-item" href="career/hiring.html">Our Hirring process</a>
</div>
</li>
<li class="nav-item" align="center">
<i class="fa fa-pencil" aria-hidden="true"></i>
<a class="nav-link" href="contect_us.html">Contact Us</a>
</li>
</ul>
</div>
<hr class="col-md-12 nav_hr">
</nav>
<!--end of navbar-->
</div>
</body>
</html>

You have to add below jQuery code in your custom js file
$('.nav-link').hover(function() {
$(this).closest('.nav-item.dropdown').siblings('.nav-item.dropdown').removeClass('show').find('.dropdown-menu').removeClass('show');
})
The issue is when you click on the dropdown link, a class named show is added to .nav-item.dropdown and .dropdown-menu, So you have to remove show class when you hover on others dropdown link
$('.nav-link').hover(function() {
$(this).closest('.nav-item.dropdown').siblings('.nav-item.dropdown').removeClass('show').find('.dropdown-menu').removeClass('show');
})
.dropdown:hover>.dropdown-menu {
display: block;
}
<!DOCTYPE html>
<html lang="en">
<head>
<title>
Welcome - Maqbool Solutions
</title>
<link rel="shortcut icon" href="../images/favion_Y13_5.ico" type="image/x-icon">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css" integrity="sha384-PsH8R72JQ3SOdhVi3uxftmaW6Vc51MKb0q5P2rRUpPvrszuE4W1povHYgTpBfshb" crossorigin="anonymous">
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.3/umd/popper.min.js" integrity="sha384-vFJXuSJphROIrBnz7yo7oB41mKfc8JzQZiCq4NCceLEaO4IHwicKwpJf9c9IpFgh" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/js/bootstrap.min.js" integrity="sha384-alpBpkh1PFOepccYVYDB4do5UnbKysX5WZXm3XxPqe5iKTfUKjNkCk9SaVuEZflJ" crossorigin="anonymous"></script>
</head>
<body>
<div class="container-fluid">
<!--main container-->
<!-- navbar-->
<nav class="navbar navbar-expand-lg navbar-custom fixed-top" id="navbar" style=" height: 80px;">
<div id="logo">
<a href="index.html">
<h3 class="logo_text">Maqbool Solutions</h3>
</a>
</div>
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<ul class="navbar-nav ml-auto" id="ul">
<li class="nav-item " align="center">
<i class="fa fa-home " aria-hidden="true"></i>
<a class="nav-link" href="index.html">Home <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item dropdown" align="center">
<i class="fa fa-cubes" aria-hidden="true"></i>
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Products
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="products/ims/ims.html">Institute Managment Systems</a>
<a class="dropdown-item" href="products/pnms/pnms.html">Pension Managment Systems</a>
<a class="dropdown-item" href="products/pms/pms.html">Property Managment Systems</a>
<a class="dropdown-item" href="products/labms/labms.html">Laboratory Managment Systems</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="products/apps/apps.html">Apps</a>
</div>
</li>
<li class="nav-item dropdown" align="center">
<i class="fa fa-cogs" aria-hidden="true"></i>
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Services
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="services/skill_service.html">Training and Skills</a>
<a class="dropdown-item" href="services/indestrial_services.html">Industry Expertise</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="services/web_service.html">Web services</a>
<a class="dropdown-item" href="services/software_service.html">Software services</a>
<a class="dropdown-item" href="services/animation_service.html">Animation services</a>
<a class="dropdown-item" href="services/graphic_service.html">Graphic services</a>
<a class="dropdown-item" href="services/game_service.html">Game services</a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="services/apps_service.html">App services</a>
</div>
</li>
<li class="nav-item dropdown" align="center">
<i class="fa fa-pencil-square-o" aria-hidden="true"></i>
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Training
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="training/soft.html">Software Designing & Development</a>
<a class="dropdown-item" href="training/design.html">Designing & Composing</a>
<a class="dropdown-item" href="training/web.html">Web Designing & Development</a>
<a class="dropdown-item" href="training/animation.html">2D&3D Animation & Development </a>
<a class="dropdown-item" href="training/game.html">Game Development </a>
<div class="dropdown-divider"></div>
<a class="dropdown-item" href="training/app.html">App Development </a>
</div>
</li>
<li class="nav-item dropdown" align="center">
<i class="fa fa-briefcase" aria-hidden="true"></i>
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
About Us
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="aboutus/hwr.html">Who we are</a>
<a class="dropdown-item" href="aboutus/our_vision.html">Our vision</a>
<a class="dropdown-item" href="aboutus/our_team.html">Our Team</a>
<a class="dropdown-item" href="aboutus/policy.html">Privacy & Policy</a>
</div>
</li>
<li class="nav-item dropdown" align="center">
<i class="fa fa-graduation-cap" aria-hidden="true"></i>
<a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Career
</a>
<div class="dropdown-menu" aria-labelledby="navbarDropdown">
<a class="dropdown-item" href="career/jobs.html">Jobs</a>
<a class="dropdown-item" href="career/inter.html">Internship</a>
<a class="dropdown-item" href="career/hiring.html">Our Hirring process</a>
</div>
</li>
<li class="nav-item" align="center">
<i class="fa fa-pencil" aria-hidden="true"></i>
<a class="nav-link" href="contect_us.html">Contact Us</a>
</li>
</ul>
</div>
<hr class="col-md-12 nav_hr">
</nav>
<!--end of navbar-->
</div>
</body>
</html>

In your code, while mouse over or click event is fired , "show" class is applied to child menu (dropdown-menu) element.
So you can override that class like:
.dropdown-menu.show {
display: none;
}
to solve your issue and to display menu items only on hover

Simple add following CSS lines:
.dropdown:hover > .dropdown-menu,
.dropdown:hover > .dropdown-menu.show {
display: block;
}
.dropdown > .dropdown-menu,
.dropdown > .dropdown-menu.show {
display: none;
}