I also think G (Authentication and Authorization) is a better answer. I cannot see clues in the wording that suggest the use of 'Confidentiality'.
Unless you consider that '...so that users of a particular department are unable to access data related to any other department' means that a member of one department could install a network sniffer to access other department's data...
But I don't think
you should suppose such scenarios if the question doesn't mention it.