Question

imperfect er/ir
yo ía nosotros/as íamos
tú ías vosotros/as íais
él/ella/usted ía ellos/ellas/ustedes ían
qué ______ (comer) frecuentemente ustedes?
comíamos
comían
comía

Explanation:

The question "¿Qué ____ (comer) frecuentemente ustedes?" is in the second - person plural (ustedes) and in the imperfect tense. According to the conjugation table for ER/IR verbs in the imperfect tense, the correct form for "ustedes" is "comían".

Answer:

comían