Final answer:
The most appropriate verb to fill in the blank is "cumplió" which means "to turn".
Step-by-step explanation:
The most appropriate verb to fill in the blank is B. cumplió. The sentence translation in English is, "Yesterday, my niece turned five years old. We celebrated with a big party." The verb "cumplir" means "to turn" in English, and it is used to indicate someone's age. In this case, it is used to express that the niece turned five years old.
Learn more about Verbs in Spanish