A physical control is the correct answer because it involves security measures that provide a physical barrier to protect assets, such as hardware, personnel, and physical infrastructure. In the context of data center security, controls like biometric locks and security guards are physical in nature because they involve tangible, real-world barriers to access. An encryption algorithm, while an important security measure, is a technical control designed to protect data confidentiality by converting readable data into an unreadable format to anyone without the key, not to prevent physical access. Managerial controls involve strategies, policies, and procedures used to direct the organization's activities regarding cybersecurity, such as security policies and compliance procedures, and do not involve physical barriers. Deterrent controls, while they aim to discourage violations, are more about the psychological impact of a possible control, not a physical measure to prevent unauthorized access.