The correct answer is C.
Complex sentences are formed from one independent clause and at least one dependent ( subordinate ) clause.
An independent clause can stand alone as a sentence and it makes a complete thought.
A dependent clause can not stand alone, even thought it has a subject and a verb.
The two clauses in a complex sentence are not structurally equal. Both clauses express related thoughts but only one acts as the foundation of the sentence.
An example : My sister smiled when I entered the room.
My sister smiled is an independent clause
When I entered the room is a dependent clause
when is the subordinating conjunction
