Which of the following encryption types protects data at rest?

Data at rest refers to information stored on a device or disk, such as a hard drive. The encryption that protects it is typically file-level encryption (or full-disk encryption).

“File-level encryption applies encryption to individual files or directories on storage media and is used to protect data at rest.” — CompTIA ITF+ Study Guide

VPN and HTTPS protect data in transit.

Plaintext means data is unencrypted, and therefore is not protective.

Therefore, A is the correct answer.