Final answer:
The correct verb to fill in the blank is 'escribiste'.
Step-by-step explanation:
The correct verb to fill in the blank is escribiste. The sentence is asking if you wrote a letter to your grandmother. The pronoun 'le' is used to indicate that the action of writing the letter is directed towards your grandmother. Therefore, the correct form of the verb 'escribir' (to write) is 'escribiste' to match the second person singular form of 'tú' (you).
Learn more about Verb conjugation in Spanish