Answer:
Ignacio: Fui de vacaciones por veinte días.
Marta: ¿Cuándo comenzaron tus vacaciones?
Ignacio: El mes pasado.
Explanation:
This dialog in English means:
Ignacio: I went on holidays for twenty days.
Marta: When did your holidays start?
Ignacio: Last month.
We can deduce his holidays started last month because the other options are incoherent. "La semana pasada" means "last week". A week has only seven days so if he had a twenty days holiday it's impossible they started last week. "Anoche" means "last night" and "anteayer" means "the day before yesterday". None of those can be possible answers for the same reason given before.
