$("#dropdown-mobile").on('change', function(){
    console.log("list item selected");
    var val = $(this).val();    

    console.log(val);
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.2/jquery.min.js"></script>
<select id="dropdown-mobile" name="select">
  <option>abc</option>
   <option>abcd</option>
</select>

您应该更改

$(document).on('change', 'option.class-x', function(){
    console.log("list item selected");
    // do whatever here
});

进入

$("#dropdown-mobile").on('change', function(){
    console.log("list item selected");
    // do whatever here
});