The correct answer is: "Once rotated, older key versions can be used for encryption until they are deleted." The statement that is NOT valid about rotating keys in the OCI Vault service is: "Once rotated, older key versions can be used for encryption until they are deleted." In the OCI Vault service, when you rotate a master encryption key (MEK), a new key version is automatically generated. However, once a key is rotated and a new version is created, the older key versions are no longer usable for encryption. The purpose of key rotation is to ensure that the encryption keys are regularly updated and that older keys are no longer used to protect sensitive data. This enhances security by minimizing the impact of potential key compromises. The other statements mentioned are valid: Both software and hardware security module (HSM)-protected MEKs can be rotated. This provides flexibility in choosing the type of MEK and ensures that key rotation can be performed regardless of the encryption method used. Each key version is tracked internally with separate unique OCIDs (Oracle Cloud Identifiers). This allows for easy management and tracking of different key versions within the OCI Vault service. In summary, the statement that is NOT valid is the one suggesting that older key versions can still be used for encryption until they are deleted. Key rotation is designed to ensure the use of the latest key version and to retire older key versions to enhance security.