bootstrap dropdown select

1<div class="dropdown">
2  <button class="btn btn-secondary dropdown-toggle" type="button" id="dropdownMenu2" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
3    Dropdown
4  </button>
5  <div class="dropdown-menu" aria-labelledby="dropdownMenu2">
6    <button class="dropdown-item" type="button">Action</button>
7    <button class="dropdown-item" type="button">Another action</button>
8    <button class="dropdown-item" type="button">Something else here</button>
9  </div>
10</div>

1<div class="dropdown show">
2  <a class="btn btn-secondary dropdown-toggle" href="#" role="button" id="dropdownMenuLink" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
3    Dropdown link
4  </a>
5
6  <div class="dropdown-menu" aria-labelledby="dropdownMenuLink">
7    <a class="dropdown-item" href="#">Action</a>
8    <a class="dropdown-item" href="#">Another action</a>
9    <a class="dropdown-item" href="#">Something else here</a>
10  </div>
11</div>

1<select class="custom-select">
2  <option selected>Open this select menu</option>
3  <option value="1">One</option>
4  <option value="2">Two</option>
5  <option value="3">Three</option>
6</select>

1<select class="selectpicker" required multiple>
2  <optgroup label="Picnic">
3    <option>Mustard</option>
4    <option>Ketchup</option>
5    <option>Relish</option>
6  </optgroup>
7  <optgroup label="Camping">
8    <option>Tent</option>
9    <option>Flashlight</option>
10    <option>Toilet Paper</option>
11  </optgroup>
12</select>
13

1<div class="dropdown">
2  <button class="btn dropdown-toggle sr-only" type="button" id="dropdownMenu1" data-toggle="dropdown">
3    Dropdown
4    <span class="caret"></span>
5  </button>
6  <ul class="dropdown-menu" role="menu" aria-labelledby="dropdownMenu1">
7    <li role="presentation"><a role="menuitem" tabindex="-1" href="#">Action</a></li>
8    <li role="presentation"><a role="menuitem" tabindex="-1" href="#">Another action</a></li>
9    <li role="presentation"><a role="menuitem" tabindex="-1" href="#">Something else here</a></li>
10    <li role="presentation" class="divider"></li>
11    <li role="presentation"><a role="menuitem" tabindex="-1" href="#">Separated link</a></li>
12  </ul>
13</div>