Final answer:
The correct verb to use is 'escribiste'.
Step-by-step explanation:
The correct verb to fill in the blank is escribiste.
The sentence is asking 'Did you write a letter to your grandmother?' and the correct response is 'You wrote a letter to your grandmother.'
The verb 'escribiste' is the second person singular form of the verb 'escribir' which means 'to write' in Spanish.
Learn more about Spanish verb conjugation