How is the form of who or whom used in the sentence?

We trying to find out whose car is parked in front of our driveway.

A: Subject

B: Direct object

C: Object of a preposition

D: Possessive

Understand the difference between who and whom. Both who and whom are relative pronouns. However, who is used as the subject of a sentence or clause, to denote who is doing something (like he or she). On the other hand, whom is used as a direct or indirect object of a verb or preposition. in this case your answer is D: Possessive