Mis hermanos la cena en la cocina. a. hacía c. hacíamos b. hacías d. hacían

All the options are conjugations of the verb hacer in the imperfect tense. This tense 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. Since mis hermanos can be replaced by ellos, which is the subject pronoun that matches the third person plural, we need to choose the conjugation of the verb hacer that also matches this person, which is hacían. Finally:

Mis hermanos hacían la cena en la cocina