The most likely answer from the choices you've provided is: B) practicing rituals and ceremonies. It looks to be some kind of old temple that would be used for that sort of thing.
A) and C) are incorrect as they're both just ridiculous and based on the size of this structure this would not be used for defending against an army, If it were to be used as "defending" Then it wouldn't have had staircases up all sides therefore it is not D). That leaves the only possible answer is B)
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