HTML

html disable first option

<select name="name">
  <option disabled="disabled" selected="selected">Select an option.</option>
  <option>A</option>
  <option>B</option>
  <option>C</option>
</select>

disable first option in select

<select>
   <option hidden>Choose</option>
   <option>Item 1</option>
   <option>Item 2</option>
</select>

Disable form first dropdown option

<!-- disable form first dropdown option -->
<script type="text/javascript">
  $( "form option:first-child" ).attr("disabled","disabled");
</script>