Answer is B. bathing suits.
You may know that a noun is a word that names a person, place or thing. Whereas, preposition is a word which comes before a noun to relate it with the noun, pronoun or with the remaining part of the sentence.
So basically a noun would function as the object of preposition.
Now, in the sentence 'Girls in bathing suits played volleyball.', the word girls is a noun, in is a preposition, bathing suits is another noun, played is a verb and volleyball is again a noun acting as an object.
Since the object of preposition is a noun that comes after a preposition, therefore the answer is B.