Nice basics question.

Odd sort of explanation though - maybe the author thought the references explained it well enough that he didn't need to say anything himself about why the actual answer is right and other options wrong. For the few people who got this wrong it might have been worth pointing out that any select statement of the general form

select col from something where col is null

can't return anything that isn't null, which immediately eliminates options 2,4,5 and 6, leaving only 1 and 3 as possibilities.