Which word is being used as the object of a preposition in this sentence?

Girls in batying suits played volleyball.

A. girls

B. bathing suits

C. in

D. volleyball

The answer is C because preposition is when is a word governing, and usually preceding, a noun or pronoun and expressing a relation to another word or element in the clause, as in "the man {on} the platform," "she arrived {after} dinner," "what did you do it {for}?"

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.