Final answer:
To fill in the blank, conjugate the verb 'servir' to match the subject 'Tú' in the preterite tense to get 'serviste'.
Step-by-step explanation:
To fill in the blanks with the preterite form of the verb, we need to conjugate the verb 'servir' to match the subject 'Tú'. In the preterite tense, the conjugation for 'servir' is 'serviste'. Therefore, the correct sentence would be: 'Tú le serviste la merienda a tu familia.'
Learn more about Conjugating verbs in Spanish