listmenu

function nn(){

if($("#a1").val() == "a"){

$("#ax").hide();

}

}
<form name="form1" method="post" action="">
  <select name="select" id="a1" onChange="nn()">
    <option value="a">a</option>
    <option value="b">b</option>
    <option value="c">c</option>
  </select>
  <select name="select3" id="a2">
  <option id="ax" value="a">a</option>
    <option value="b">b</option>
    <option value="c">c</option>
  </select>
</form>