A. swim because B doesn't make sense, neither does D. It could be A or C, but I'd pick A. I did this test before and I picked A, and got 80 % so good luck ;)
They all like the beach, but <em>none of them </em><em>swim</em> in the ocean. After none of + a plural word (none of the shops, none of them etc.) it's possible to use a <em>singular or plural verb</em>, but a <em>plural verb is more usual</em>. None of the shops were (or was) open. Also, none + of it's quite usual as a subject. B: past of swim is swam. C: past participle of swim is swum
