Which sentence is a complex sentence?

Ashlee will bring chips and salsa to the party.

My dad, however, does not think skateboarding is safe.

Emil wants to go to the concert, but he doesn't have a ride.

Although I am tired, I will finish my homework.

Although is a complex transition word and fitted properly therefore

D. "Although I am tired, I will finish my homework", is the answer

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.

1) * Although I am tired,

2) I will finish my homework