Which of the following structures consists of one main clause and at least one subordinate clause?

A complex sentence is defined by having one main clause and at least one subordinate clause. The main clause can stand alone as a complete thought, while the subordinate clause adds additional information but cannot stand alone. For example, in the sentence "Although it was raining, we decided to go for a walk," "we decided to go for a walk" is the main clause, and "Although it was raining" is the subordinate clause. This structure allows for more nuanced expressions and relationships between ideas, showcasing the complexity of thought in writing.

A simple sentence consists of just one main clause without any subordinate clauses, while a compound sentence has at least two main clauses joined by a coordinating conjunction, and a compound-complex sentence has at least two main clauses and at least one subordinate clause. Therefore, the defining characteristic of a complex sentence—the presence of at least one subordinate clause in addition to the main clause—makes it the correct choice in this case.