ms-Dropdown

<div class="ms-Dropdown" id="ID_dropdown" tabindex="0"> 
<label class="ms-Label">My Label</label> 
<i class="ms-Dropdown-caretDown ms-Icon ms-Icon--caretDown"></i> 
<select class="ms-Dropdown-select"> 
   <option>Option 1</option> 
   <option selected>Option 2</option> 
   <option>Option 3</option> 
</select> 
</div> 

<script type="text/javascript"> 
$(document).ready(function() { 
   if ($.fn.Dropdown) { 
      $('.ms-Dropdown'}.Dropdown();  
   } 
}); 
</script>