Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<select id="pets" size="3">
  <option>Cat</option>
  <option>Dog</option>
  <option>Horse</option> 
</select>

<p>Click the button to disable the third option (index 2) in the dropdown list.</p>

<button onclick="myFunction()">Disable Option</button>

<script>
function myFunction() {
    var x = document.getElementById("pets").options[2].disabled = true;
}
</script>

</body>
</html>