Which of the following is a complex sentence?

A complex sentence is characterized by the presence of one independent clause and at least one dependent clause. In this case, the correct choice includes the dependent clause "Although it was raining," which cannot stand alone as a complete thought, thus relying on the independent clause "we went for a walk" to provide a complete idea.

The structure showcases the relationship between the clauses, indicating that the action of going for a walk happened despite the rain. This connection between the dependent and independent clauses is what distinguishes a complex sentence from others, like the first choice, which is a compound sentence made up of two independent clauses linked by a conjunction. The second and fourth choices consist of single independent clauses, which do not meet the criteria for complex sentences.