$(document).ready(function() {
$("#gate option[value='Gateway 2']").prop('selected', true);
// you need to specify id of combo to set right combo, if more than one combo
});
$('select').on('change', function() {
alert( this.value );
});$("#mySelectElementID").val("my_new_value"); //change selected option/value with jQuery
$("div.id_100 select").val("val2");
<select onchange="getval(this);">
<option value="1">One</option>
<option value="2">Two</option>
</select>
Run code snippetfunction getval(sel)
{
alert(sel.value);
}