Final answer:
The correct form of the verb llamar to use in the sentence is llamaba.
Step-by-step explanation:
The correct form of the verb llamar to use in the given sentence is llamaba.
When telling a story in the past, it is common to use the imperfect tense. In this case, the imperfect form of llamar is llamaba.
Using the appropriate form of the verb completes the sentence: Había una vez una niña que se llamaba Caperucita Roja.
Learn more about Spanish verb conjugation