how to get a list of all top level pages

by 

$query_args = array(
    'post_type'   => 'page',
    'post_status' => 'publish',
    'parent'      => 0,
);

$pages = get_pages( $query_args );

Function get_pages() accepts parameter with parent. Keeping it 0 (zero) will give us first level pages. In the example $pages will contain the first level pages. Use loop for $pages and you can use it in option as you require.