Cuáles" is for plural words whereas Cuál refers to singular objects/words. This essentially means that because of the usage of the word, "frutas," which is plural, then B would be the correct answer.
The correct answer to your question is option B. "Cuàles."
The question is asking "What are your favorite fruits?". "Fruits" is a plural noun so the correct answer is cuàles. If the question was asking "What is your favorite fruit?", then your answer would be option A since it is a singular noun.
