Encrypting hard drives is a direct implementation of confidentiality, one of the three pillars of the CIA Triad emphasized in CompTIA Security+ SY0-701. Full disk encryption ensures that if a laptop, workstation, or server drive is stolen or accessed without authorization, the data remains unreadable without the decryption key.

This controls unauthorized disclosure and protects sensitive business, financial, and personal information. Drive encryption is widely required for compliance frameworks such as HIPAA, PCI-DSS, and GDPR.

Integrity (A) refers to preventing unauthorized modification of data, which encryption alone does not guarantee. Authentication (B) confirms user identity, such as passwords or biometrics, but is unrelated to data-at-rest protection. Zero Trust (C) is an architectural model requiring constant verification; it is not a control for hard drive encryption.

Since the sole purpose of encrypting storage is to ensure data confidentiality, the correct answer is D.