The company requires enabling server-side encryption with S3-managed keys (SSE-S3) for all objects, both new and existing, in the most operationally efficient manner. First, enabling default encryption on the S3 buckets ensures all new objects are automatically encrypted with SSE-S3 upon upload. However, this setting does not retroactively encrypt existing objects. For the millions of existing objects, S3 Batch Operations is the most operationally efficient solution. It is a managed service designed for performing large-scale bulk operations. A Batch Operations job can be configured to invoke the S3 Put Object - Copy API action on all existing objects, copying them in place. During this copy operation, the objects will be encrypted according to the bucket's new default encryption policy (SSE-S3), fulfilling the requirement without needing custom scripts or manual intervention.

B. This is incorrect because the requirement specifies using SSE-S3, not SSE-KMS. Additionally, using the AWS CLI for millions of objects is less operationally efficient than S3 Batch Operations.
C. While this approach would work, using the AWS CLI with a copy command is not the most operationally efficient method for millions of objects compared to the fully managed S3 Batch Operations service.
D. This is incorrect because the requirement is to use SSE-S3, not SSE-KMS. Although it correctly identifies S3 Batch Operations as the efficient tool, it proposes the wrong encryption type. ---

1. AWS S3 User Guide - Setting default server-side encryption behavior for Amazon S3 buckets: "When you enable default encryption for a bucket, the following is true: ... Existing objects in the bucket are not encrypted. To encrypt existing objects, you can use Amazon S3 Batch Operations." This confirms that default encryption is not retroactive and points to S3 Batch Operations as the solution for existing objects.

2. AWS S3 User Guide - Encrypting objects with Amazon S3 Batch Operations: "To encrypt a group of unencrypted objects, you can create a Batch Operations job. The Batch Operations job runs a copy operation that copies the objects in place... When you create the job, you specify that the new object copies must be encrypted." This directly supports using S3 Batch Operations with a copy-in-place action to encrypt existing objects.

3. AWS S3 User Guide - S3 Batch Operations basics: "With S3 Batch Operations, you can perform large-scale batch operations on Amazon S3 objects. S3 Batch Operations can run a single operation on lists of S3 objects that you specify... Amazon S3 tracks progress, sends notifications, and stores a detailed completion report of all actions." This highlights the operational efficiency of the managed service over manual scripting.

4. AWS S3 User Guide - Protecting data using server-side encryption with Amazon S3-managed encryption keys (SSE-S3): "When you use Server-Side Encryption with Amazon S3-Managed Keys (SSE-S3), each object is encrypted with a unique key... As an additional safeguard, it encrypts the key itself with a root key that it regularly rotates." This defines the specific encryption method (SSE-S3) required by the question.