Mandatory vacation policies are principally designed to act as a preventative and detective control. When employees are away from their duties, others must fill in and handle their responsibilities, potentially revealing any irregularities, fraud, or unauthorized activities that the absent employee may have been concealing. It's an opportunity for the organization to audit and verify the integrity of the employee's work. Answer A is correct because it directly addresses the primary security benefit of detecting fraudulent activities or policy violations that might otherwise go unnoticed. Answer B is incorrect as it misleads by focusing on productivity, which isn't directly related to the security aspect of the mandatory vacation. Answer C is incorrect because decreasing reliance on a single employee does not directly relate to revealing fraud or misconduct, it generally addresses continuity and operational dependency. Answer D is incorrect because it diverts attention to cost-reduction, which is not a primary security concern associated with mandatory vacation policies.