Answer:
jugaba
Step-by-step explanation:
You need to conjugate the infinitive jugar in the yo form for the imperfect tense. Here are the verb endings for the regular imperfect:
yo -aba
tú -abas
él/ella/usted -aba
nosotros -abamos
vosotros -abais
ustedes -aban
We need it in the yo form, so we take the -ar off jugar and add -aba to get jugaba.