Data encryption is the correct answer because it ensures that 'Restricted' data is unreadable to unauthorized individuals even if they manage to bypass other security measures. While strict file-level permissions are important, they do not protect data from being intercepted or accessed by sophisticated attackers. Network segmentation is critical as well but does not protect data at rest, and audit logs provide a record of access but do not prevent unauthorized data viewing or manipulation. Data encryption, when properly implemented, will guard against both unauthorized access and disclosure in those scenarios.