Just use the array of options, to see, which option is currently selected.
$options = array( 'one', 'two', 'three' );
$output = '';
for( $i=0; $i<count($options); $i++ ) {
$output .= '<option '
. ( $_GET['sel'] == $options[$i] ? 'selected="selected"' : '' ) . '>'
. $options[$i]
. '</option>';
}
Sidenote: I would define a value to be some kind of id for each element, else you may run into problems, when two options have the same string representation.