The city was kept . . . . . . . night curfew for several days.

The correct answer is C. under.

A curfew is a set of rules that restrict people’s movement, especially at night. The city was kept under a night curfew for several days, which means that people were not allowed to be outside during the night.

The other options are incorrect because they do not accurately describe the situation. Option A, on, is incorrect because it suggests that the city was located on a night curfew. Option B, at, is incorrect because it suggests that the city was experiencing a night curfew at a specific time. Option D, over, is incorrect because it suggests that the city was finished with the night curfew.

I hope this helps!