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.