The question emphasizes “MOST securely” and involves a multi-account backup strategy with a centralized backup account. AWS Backup best practice for high-security environments is to use customer managed KMS keys for encrypting backup vaults rather than AWS managed keys. Customer managed keys provide tighter control over key lifecycle, key policies, and cross-account access.

Option A creates encrypted backup vaults in both the source and centralized accounts, each protected by a customer managed KMS key in that account. AWS Backup is then configured to create full EC2 backups (AMIs) and copy them to the centralized backup vault. This design ensures:

Encryption at rest in both accounts.

Independent key control and policies per account.

Secure cross-account backup copy behavior governed by explicit KMS key grants and vault access policies.

Option B incorrectly uses the source account’s KMS key to encrypt vaults in both accounts, which is not the recommended pattern; each account should manage its own CMK. Options C and D rely partially or fully on AWS managed keys, which are less appropriate when the requirement explicitly stresses maximum security and control.

Therefore, Option A best satisfies both the RPO/RTO goals (daily AMI backups with rapid restore) and the strongest encryption posture.