this code:
<?php if (!(strcmp("Commercial", (isset($_GET["invalid"])?ValidatedField("contact","Section"):"")))) {echo "selected=\"selected\"";} ?>
should add the option URL perameter as the false part:
<?php if (!(strcmp("Commercial", (isset($_GET["invalid"])?ValidatedField("contact","Section"):(isset($_GET['option'])?$_GET['option']:"")))) {echo "selected=\"selected\"";} ?>