Final answer:
The appropriate preterite form of the verb estar to fill in the blank is estuvo.
Step-by-step explanation:
The appropriate preterite form of the verb estar to fill in the blank is estuvo.
Preterite is the past tense in Spanish, and it is used to talk about completed actions in the past. In this sentence, the action of being at home occurred in the past while we were going to the store, so we use the preterite form of estar, which is estuvo for the third person singular form (usted).
Example: Mientras yo iba a la escuela, mi hermana estuvo en casa.
Learn more about Spanish verb conjugation