GMAT Sentence Correction (SC) | Re: QOTD: Since 1986 when the Department of Labor

HiGMATNinja ,

Though we have no way to disagree with OA, I have the below doubt with option D

In "Fees of investment officers" , investment officers is the object of the preposition "of". And it has been much discussed in other posts that object of a preposition can't act as Subject of a sentence. So how is it that "they"'s antecedent is an object of a preposition?