Final answer:
The correct imperfect verb for the sentence is 'estaba.' It indicates an ongoing action in the past.
Step-by-step explanation:
For the sentence 'Todos los sábados yo ___ en la casa de los abuelos,' the correct imperfect verb is 'estaba.' The verb 'estaba' is the imperfect form of the verb 'estar' which means 'to be.' In this context, it indicates an ongoing action in the past. The sentence would be translated as 'Every Saturday, I was at my grandparents' house.'
Learn more about Imperfect verb usage in Spanish