The question "How often do you visit your grandparents?" is asking about the frequency of the action (visiting grandparents). The answer "Once a week" specifies that the action is done weekly, indicating the frequency of the visits.
So, when someone responds with "Once a week", it directly answers the question by providing a clear frequency (once every week) for the action of visiting grandparents.
The question is structured to inquire about how frequently something happens, and the response "Once a week" gives the exact timeframe or schedule for the activity.
