Which of the following is a complex sentence? a. after henry submitted his essay, he realized that he hadn't proofread the last page. b. identity theft is a growing problem, especially among seniors. c. we didn't have enough potatoes for dinner, so we served rice instead. d. jim tried to watch a video on a device connected to a crowded wireless network; as a result, the video stuttered and froze.

The complex sentence is A.  After Henry submitted his essay, he realized that he hadn't proofread the last page.

A complex sentence is a sentence that contains an independent clause and a dependent clause. In contrast to an independent clause, a dependent clause cannot stand alone to form a complete sentence. Moreover, dependent clauses are always introduced by a subordinating conjunction. In this sentence, the dependent clause is "After Henry submitted his essay", the subordinating conjunction is "after" and the independent clause, which can stand by itself, is "he realized that he hadn't proofread the last page".