Right answer:
b. preferías
Preferías is the conjugation of the verb preferir for the second person singular in the imperfect tense. We use this tense when talking about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past. This verb has regular conjugations in the imperfect. To conjugate it, take away the ir ending from the verb preferir and add ías to the end of the word. So:
Tú preferías escuchar la radio en la sala