Complete the sentence
¿Cuántos años ____ tú cuando aprendiste a nadar?
Answer: d. tenías
¿Cuántos años tenías tú cuando aprendiste a nadar?
How old were you when you learned to swim?
Step-by-step explanation: In a compound sentence (one that contains two or more verbs), the verb that provides background for a past action is in the imperfect. The verb that indicates the age must be in the imperfect as background to learn to swim, which is in the preterite. In this sentence, the subject is "tú", that is a second-person in the singular, so the verb must be conjugated in this form: tú tenías.