this

<p id="demo"></p>

<script type="text/javascript" src='jquery-1.8.2.js'></script>
<select class="form-control" name="aset" id="aset">
<optgroup class="form-control" label="">

<?php
//include('../../page/koneksi.php');
error_reporting(0);
    $user = "root";
    $pass = "";
    $host = "localhost";
    $db   = "latihan";

   $db_server = mysql_connect($host, $user, $pass);
	mysql_select_db($db) ;

//$npk = $_SESSION['npk'];
$sql = mysql_query("SELECT
            *
        FROM
            ujian_ket_pengemudi");
    if(mysql_num_rows($sql) != 0){
        while($data = mysql_fetch_assoc($sql)){
            echo "<option value='$data[gol_sim]'>$data[gol_sim]</option>";
        }
    }
    ?>

</optgroup>
</select>

<script type="text/javascript">
//var htmlobjek;
$(document).ready(function(){
	$("#aset").change(function(){
    var xaset = $("#aset").val();
	document.getElementById("demo").innerHTML = xaset;
  });
});
</script>