I think the easiest way is selecting to set val(), but you can check the following. See How to handle select and option tag in jQuery? for more details about options.
$('div.id_100 option[value="val2"]').prop("selected", true);
$('id_100').val('val2');
Not optimised, but the following logic is also useful in some cases.
$('.id_100 option').each(function() {
if($(this).val() == 'val2') {
$(this).prop("selected", true);
}
});