Bootstrap 4 

<!doctype html>
<html lang="en">
  <head>
    <!-- Required meta tags -->
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
    <title>Bootstrap 4</title>
    <style>
        html { -webkit-box-sizing: border-box; -moz-box-sizing: border-box; box-sizing: border-box; } 
        *, *:before, *:after { -webkit-box-sizing: inherit; -moz-box-sizing: inherit; box-sizing: inherit; }
    </style>
    <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://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/1.12.9/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>
</head>
<body>
...
</body>
</html>

Bootstrap 4 - Forms

Bootstrap provides several form control styles, layout options, and custom components for creating a wide variety of forms.

https://v4-alpha.getbootstrap.com/components/forms/

Bootstrap 4 - Modal

https://v4-alpha.getbootstrap.com/components/modal/

Bootstrap 4 - DropDowns

https://v4-alpha.getbootstrap.com/components/dropdowns/

Bootstrap Select

<div class="selectWrapper">
    <select>
        <option>Lorem</option>
        <option>Parturient</option>
        <option>Euismod</option>
    </select>
</div>
.selectWrapper {
    width: 100%;
    overflow: hidden;
    position: relative;
    border: 1px solid #bbb;
    border-radius: 2px;
    background:#FFFFFF url('data:image/svg+xml,%3Csvg%20xmlns%3D%22http%3A%2F%2Fwww.w3.org%2F2000%2Fsvg%22%20width%3D%2211%22%20height%3D%2211%22%20viewBox%3D%220%200%2011%2011%22%3E%3Cpath%20d%3D%22M4.33%208.5L0%201L8.66%201z%22%20fill%3D%22%2300AEA9%22%2F%3E%3C%2Fsvg%3E') right 13px center no-repeat;
}

    .selectWrapper select {
        padding: 12px 40px 12px 20px;
        font-size: 18px;
        line-height: 18px;
        width: 100%;
        border: none;
        box-shadow: none;
        background: transparent;
        background-image: none;
        -webkit-appearance: none;
        outline: none;
        cursor: pointer;
        -moz-appearance: none;
        text-indent: 0.01px;
        text-overflow: ellipsis;
    }

Bootstrap-Select

Bootstrap-select is a jQuery plugin that utilizes Bootstrap's dropdown.js to style and bring additional functionality to standard select elements.

https://silviomoreto.github.io/bootstrap-select/

<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.12.4/css/bootstrap-select.min.css">

<!-- Latest compiled and minified JavaScript -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/bootstrap-select/1.12.4/js/bootstrap-select.min.js"></script>

Bootstrap 3

<!DOCTYPE html>
<html>
    <head>
         <meta name="viewport" content="width=device-width, initial-scale=1">
         <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
         <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
         <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
    </head>
    <body>
        <div class="container">
             <h2>Dropdowns</h2>
             <p>The .dropdown class is used to indicate a dropdown menu.</p>
             <p>Use the .dropdown-menu class to actually build the dropdown menu.</p>
             <p>To open the dropdown menu, use a button or a link with a class of .dropdown-toggle and data-toggle="dropdown".</p> 
             <div class="dropdown">
                 <button class="btn btn-primary dropdown-toggle" type="button" data-toggle="dropdown">Dropdown Example
                 <span class="caret"></span></button>
                 <ul class="dropdown-menu">
                     <li><a href="#">HTML</a></li>
                     <li><a href="#">CSS</a></li>
                     <li><a href="#">JavaScript</a></li>
                 </ul>
             </div>
        </div>
    </body>
</html>