Amazon RDS does not support enabling encryption at rest on an existing unencrypted DB instance. To encrypt an existing RDS instance's data at rest, the recommended method is to:

Take a snapshot of the unencrypted DB instance.

Create an encrypted copy of the snapshot using AWS KMS. This encrypted snapshot contains the existing data encrypted at rest.

Restore a new DB instance from the encrypted snapshot. This new instance will have encryption at rest enabled.

Additionally, to manage encryption keys securely, companies can use customer managed keys (CMKs) in AWS Key Management Service (KMS). CMKs provide greater control over key management policies, rotation, and usage permissions compared to default AWS managed keys. Using a CMK allows customization of access control and auditability.

Option A is incorrect because you cannot enable encryption directly on a snapshot; you must create an encrypted copy. Option C is invalid because encryption cannot be enabled by modifying an existing instance's configuration. Option D refers to the default AWS managed key, which is less flexible than customer managed keys.

References:

Encrypting Amazon RDS Resources (https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Overview.Encryption.html)

Copying an Encrypted Snapshot (https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_CopySnapshot.html)

AWS KMS Customer Master Keys (https://docs.aws.amazon.com/kms/latest/developerguide/concepts.html)

AWS Well-Architected Framework --- Security Pillar (https://d1.awsstatic.com/whitepapers/architecture/AWS_Well-Architected_Framework.pdf)