Right answer:
c. estudiabas
Estudiabas is the conjugation of the verb estudiar for the second person singular (tú - you in English) in the imperfect. The imperfect 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. The verb estudiar is regular. For regular ar verbs, take away the ar ending and add abas to the end of the word for the second person singular.