Correct answer:
todo el tiempo
This is the correct option because this sentence stands for the imperfect. So imperfect tenses talk about action that occurred regularly or frequently or that were in progress in a moment in the past. So todo el tiempo means all the time and this tells us that the sentence were in progress in a moment in the past. The other options talk about a specific moment in the past. Finally:
En las vacaciones con mi amiga Laura nos bañábamos todo el tiempo en la piscina de Marcos.