The complex sentence is A. After Henry submitted his essay, he realized that he hadn't proofread the last page.
<u>A complex sentence is a sentence that contains an independent clause and a dependent clause</u>. 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,<u> 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"</u>.
