Right answer:
buscábamos
In general, the imperfect is used to talk about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past. It is also used to tell time, talk about dates, give a person's age, and describe features, conditions, and feelings in the past. Since nosotros is the subject pronoun that matches the first person plural, then we must conjugate the verb buscar for this person, too. This conjugation is buscábamos in the imperfect. Finally:
Nosotros buscábamos la llave