This is not new but I would like it to be posted on my site. So here is the jQuery code to trigger the event when user selects the HTML option box.

<script type="text/javascript">
jQuery(document).ready(function($){
 $("#yourOptionID").change(function (){
 var selVal = $("#yourOptionID").val();
 if(selVal != null){
 //Add your code
 }
 else {
 //Add your code
 }
 });
 });
 </script>

First you need to put this code into your header.php file.  Probably your header.php file already had wp_head(); .  If so just add below code above the wp_header();

[code]wp_enqueue_script("jquery");[/code]

After that you can call your own jQuery Javascript file.

To be super-safe, you can put jQuery into “no conflict” mode and use a different shortcut for jQuery. In this case “$jQ” instead of the default “$”. The standard “$”, for example, can conflict with Prototype. Here is an example of a safe bit of jQuery JavaScript:

e.g: var $jQ = jQuery.noConflict();