I took the dog for a walk in the park because he had been shut in the house all day.[___]

The sentence has one independent clause: “I took the dog for a walk in the park.” It also has a dependent clause, “because he had been shut in the house all day,” which begins with the subordinating word “because” and cannot stand alone as a complete sentence. A sentence with an independent clause and a dependent clause is complex, so the correct choice is complex.

AI-written and checked against the verified answer.