The object of the preposition in sentence 7 is, option D. There are no prepositions in sentence 7. A preposition is a word that is used to express the relation between a word to another word, clause of phrase. An object of the preposition is the noun that is referred by the preposition. In the given sentence, there is no preposition; therefore, there is no object of the preposition identified as well. Examples of prepositions are: about, in, on, at, about, under, before, etc.