<script language="javascript">
function show(val){
alert(val); // >>>b
}
</script>
<form id="form1" name="form1" method="post" action="">
<select onchange="show(this.options[this.options.selectedIndex].value)">
<option value="a">1</option>
<option value="b" selected="selected">2</option>
<option value="c">3</option>
<option value="d">4</option>
</select>