The correct answer is B. Erin and Maria are going to the mall, and John is going to the movies. A compound sentence is when there are two phrases that could stand alone as two separate sentence, but are joined together by a conjunction (usually "and"). So, "Erin and Maria are going to the mall," can work as one full sentence, and "John is going to the movies" can also work as one full sentence. They are conjoined in one sentence by the word "and," making this a compound sentence.
