To answer this question; First of all, we need to be clear of the types of existing sentences that exist such as simple sentences, compound sentences and complex compound sentences. We must also need to discriminate between dependent and independent clauses. When we read sentences A, B, and C, we can observe that they have complete meanings. What I mean with this is that they all contain a main subject and a verb. However, if we carefully read sentence letter C, we observe that it has a dependent clause in between that is, a fragment that cannot stand on its own and that’s why it needs something else in order to have a complete meaning. (THAT WAS PARKED UNDER THE TREE). Needless to say that this phrase begins with a relative pronoun.
So the correct answer for this question is letter C.
