The correct answer is d. "Tabitha and Simon had a busy day, because they started at the zoo, and then they went to the art museum for the rest of the afternoon."
Option d is a compound sentence because it contains two independent clauses joined by the coordinating conjunction "and." The two independent clauses are "Tabitha and Simon had a busy day" and "they started at the zoo, and then they went to the art museum for the rest of the afternoon."