JSFiddle - React, Tailwind, and code Playground

HTML

<input type="checkbox" class="convocati" value="A">A
<input type="checkbox" class="convocati" value="B">B
<input type="checkbox" class="convocati" value="C">C
<input type="checkbox" class="convocati" value="D">D
<select name="S1" class="convatiSelect">
   <option value=""></option>
</select>
 <select name="S2" class="convatiSelect">
   <option value=""></option>
</select>

JavaScript

$(document).ready(function(){
$(".convocati").click(function(){
   var thisVal = $(this).val();
   debugger;
   if ($(this).is(":checked"))
   {
      $(".convatiSelect").append($("<option>").attr('value', thisVal).html(thisVal));
    }
   else{
     var option = $(".convatiSelect").find("option[value='" + thisVal + "']").remove();
   }
})
})