Answer: (A) traje de baño
Explanation:
You can look at this from a translation stand point or from a “logic” standpoint.
If you look at it from a translation stand point, the explanation is simply that a bathing suit is called a “traje de baño” in spanish. “Traje” means suit and “de baño” technically means “for bathing”.
If you look at it from a “logic” sort of standpoint, you can observe that the article preceding the missing word, “un” is a singular article. So, you can eliminate (B) since it is plural (in general, plural words in spanish tend to end in “s”).
Then, you’d have only A, C, and D to choose from. To discern which of these is the right answer, you can consider the fact that, for the most part, when a term in English is made up of more than one word, its translation in Spanish will be made up of more than one word.
The only choice that includes more than one word is (A), so you could get to your answer in this way too.
Hope this helps!