how can check selected item in asp.net drop down list?

Question

I'm new in asp.net. i wrote script for checking the value of asp.net drop down list(below code):

     <script>
    $(document).ready(function () {
        debugger;
        $('#<%=offermessage.ClientID%>').on('change', function () {
            var a = $(this).val();

         if (a == 9) {
             $('#dealer_name').fadeIn();
             $('#vin_number').fadeOut();

         }
         if (a == 10) {
             $('#dealer_name').fadeIn();
             $('#vin_number').fadeIn();
         }
         if (a == 11) {
             $('#vin_number').fadeIn();
             $('#dealer_name').fadeOut();
         }
         if (a == 12) {
             $('#dealer_name').fadeOut();
             $('#vin_number').fadeOut();
         }
         if (a == 13) {
             alert("لطفا موضوع پیشنهاد و شکایت را انتخاب نمایید.")
         }
     });

 });
</script>

now i need to write an script that checked the value when i want to submit the form.below script written and function return in submit button .

     <script>
    function subjectvalidate(offermessage) {
        $('#<%=offermessage.ClientID%>').on('select', function () {
            var a = $(this).val();
            if (a == 13) {
                alert("لطفا موضوع پیشنهاد و شکایت را انتخاب نمایید.")
                return false;
            }
        }
        )};
</script>

but it's not work .any body can help me ??? thanks