Try this
$(document).ready(function () {
$('#mainSelect').on('change', function () {
$("select[name^=contact_subcategory] option").prop("selected", "");
var name = $(this).val(); alert(name);
$(name).each(function() {
$("select[name=contact_subcategory_" +this + "] option").prop("selected", "selected");
})
});
});