Your answer would be that the sentence that is a complex sentence is the following one: Although I am tired, I will finish my homework.
Explanation:
A complex sentence has one independent clause and at least one dependent clause. The difference between an independent clause and a dependent one is that the former can stand alone as a sentence as it represents a complete thought (see 1). As you can see, 1 fails to stand alone as a sentence because it does not represent a complete idea. On the contrary, "I will finish my homework" (2) do so because it is a complete sentence: it has a subject and a predicate and does not depend on any other element.
