Well, we know that the question is asking what we need to go to water (the beach or pool) so, we can rule out 1 because we don't wear that to the beach or pool, we can rule out 2 because no one wears a sweatshirt to the beach or pool, and lastly, we can rule out 3 because no one wears a sweater to the beach or pool! The answer you are looking for is 4, or el traje de baño (the swimsuit)
Hope this helps!