Two business units in separate AWS accounts share sensitive documents. Each account has an S3 bucket and the company configured low-way replication between the buckets. Neither bucket currently has encryption at rest enabled, and policy requires SSE-S3. The buckets contain millions of objects. What is the most operationally efficient way to enable SSE-S3 for existing objects and meet the policy?

The correct option is to enable SSE-S3 on both S3 buckets and use S3 Batch Operations. Enabling SSE-S3 on the buckets ensures new objects are encrypted according to policy. S3 Batch Operations is the most operationally efficient way to encrypt millions of existing objects in place. It can perform a COPY operation on existing objects, specifying the desired encryption, without needing to download and re-upload them manually. Using SSE-KMS (options B and D) is not required by the policy, which specifically states SSE-S3. While SSE-KMS offers more control, it introduces additional complexity and cost with KMS key management. Manually running S3 copy commands with the AWS CLI (option C) for millions of objects would be extremely time-consuming and inefficient compared to S3 Batch Operations.