Final answer:
The appropriate preterite form of the verb estar in the given sentence is 'estuvo'.
Step-by-step explanation:
The appropriate preterite form of the verb estar in the given sentence is estuvo. The sentence reads, "Mientras nosotros íbamos a la tienda, usted, estuvo en nuestra casa." This means, "While we were going to the store, you were at our house." The preterite form of estar is used to describe actions that occurred in the past and are completed.
Learn more about Preterite form of the verb estar