Right answer:
a. comíamos
Comíamos is the conjugation of the verb comer for the third person plural in the preterite tense. The preterite tense is used to describe actions that took place or were completed at a certain point in the past. So comer is an er regular verb. To conjugate this verb for the third person plural in the preterite tense, take away the er ending and add íamos to the end of the word. So:
Mi familia y yo comíamos la cena todos los días a las seis.