Final answer:
In Spanish, the preposition 'a' is used when asking about time.
Step-by-step explanation:
In Spanish, the preposition used to ask about time is 'a'. For example, if you want to ask 'At what time does the party start?', you would say '¿A qué hora empieza la fiesta?'. Here, 'a' is used to indicate 'at' in relation to time.
Learn more about prepositions in Spanish