Answer: b) En los conciertos de 2004, a veces no te gustaba la voz de la cantante.
Translation: At the 2004 concerts, sometimes you didn't like the singer's voice.
Step-by-step explanation:
The direct object of the verb "gustar" is "la voz", which is a singular noun, so the verb must be conjugated in this form: gustaba.
Spymore