Final answer:
The correct answer is D. It contains a dependent adverbial clause introduced by 'when'.
Step-by-step explanation:
The correct answer is D. Mark expects the photo to arrive when the letter arrives. This sentence contains a dependent adverbial clause introduced by the subordinating conjunction 'when'. A complex sentence consists of an independent clause and at least one dependent clause. In this case, the independent clause is 'Mark expects the photo to arrive', and the dependent adverbial clause is 'when the letter arrives'.
Learn more about Complex sentences