First of all, you need to know what a compound-complex sentence is. It is a type of a sentence that contains at least TWO independent clauses, and at least ONE dependent clause. A, B. and D are incorrect because all of them have only one independent clause, and there have to be at least two.
Therefore, the correct answer is C. Mariella and her sisters went to the movies, and as they sat down, Mariella reminded her sisters to turn off their cellphones.