B is the answer, why? As stated, there are 3 persons in the sentence namely: Holly, Molly and Polly and the use of 2 verbs which are sat and talked. In this sentence, it is indicated that the 3 girls sat together and talked about their friends. It clearly shows that the sentence is compound. As a brief description of a compound sentence, it has more than one subject or predicate present in the sentence and the example sentence above is considered as a compound sentence.
