Answer: "While you were sleeping, I watched a movie" is a complex sentence.
Step-by-step explanation: A complex sentence is made up of an independent clause and a dependent clause. While an independent clause can stand alone as a sentence, a dependent clause cannot do this because it does not express a full idea. Moreover, dependent clauses are always introduced by a subordinating conjunction. "While you were sleeping, I watched a movie" is a complex sentence because it includes a dependent clause ("While you were sleeping") and an independent clause ("I watched a movie"). "While you were sleeping" is a dependent clause because it does not express a complete idea and it has been introduced by the subordinating conjunction "while".