Final answer:
The correct verb to fill in the blank is 'tienes' as it matches with the pronoun 'tú'.
Step-by-step explanation:
The correct verb to fill in the blank in the sentence is tienes. The sentence is asking, "How old are you?" In Spanish, the verb tener is used to express age. Since the subject of the sentence is "tú" (you), we use the form of the verb that matches with the pronoun, which is tienes.
Learn more about Verb conjugation in Spanish