The best control to meet the Statement on Standards for Attestation Engagements 18 (SSAE-18) confidentiality category is storage encryption. SSAE-18 is a standard that defines the requirements and guidance for performing attestation engagements on service organizations, such as cloud providers, data centers, or payroll processors. SSAE-18 requires the service organizations to provide a report on the design and effectiveness of their controls over the security, availability, processing integrity, confidentiality, or privacy of the services they provide to their customers. The confidentiality category refers to the protection of the information that is designated as confidential by the service organization or its customers, and that is transmitted, stored, or processed by the service organization. Storage encryption is a control that encrypts the data at rest, such as in hard drives, databases, or backups, and that prevents unauthorized or malicious access, modification, or disclosure of the confidential information. Data processing, file hashing, or data retention policy are not the best controls to meet the SSAE-18 confidentiality category, as they are not directly related to the protection of the confidential information at rest. Data processing is a control that transforms or manipulates the data for a specific purpose, such as analysis, reporting, or validation. File hashing is a control that generates a unique and fixed-length value for a file, and that verifies the integrity or authenticity of the file. Data retention policy is a control that defines the rules and procedures for retaining, storing, or disposing of the data, and that complies with the legal, regulatory, or contractual obligations. References: Official (ISC)2 Guide to the CISSP CBK, Fifth Edition, Chapter 1: Security and Risk Management, page 71.