Question

imperfect ar

yo aba nosotros/as íbamos
tú abas vosotras/as abais
él/ella/usted aba ellos/ellas/ustedes aban

( ) (jugar) mucho cuando eras niño tú?

jugaban
jugabas
jugaba

Explanation:

The subject of the question is "tú" (you - informal singular). In the imperfect tense for "tú" with AR - verbs, the ending is "abas". The verb "jugar" (to play) becomes "jugabas" in the imperfect for "tú".

Answer:

jugabas