Answer: The option that presents a complex sentence is D. After you install the security software, your computer will run smoothly.
Explanation: A complex sentence is a sentence that includes an independent clause and one or more dependent clauses. A dependent clause is introduced by a subordinating conjunction and it cannot stand alone as a sentence, whereas an independent clause can stand by itself because it expresses a complete thought. In sentence D., the dependent clause is "After you install the security software" and the independent clause is "Your computer will run smoothly".
