Right answer:
estaba
Estaba is the conjugation of the verb estar that matches the first person singular, which is yo, in the imperfect tense. This tense is used when talking about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past. Finally, we can write this sentence as follows:
Todos los sábados yo estaba en la casa de los abuelos
Whose translation is:
I was in my grandparent's house every Saturday
