The correct answer is: Enable default encryption on the existing S3 bucket and use S3 Inventory to generate a list of unencrypted objects. Then run an S3 Batch Operations job using the copy operation to apply encryption to existing files.

This approach automates the process with minimal manual intervention. By enabling default encryption, all future uploads are encrypted automatically. For existing files, S3 Inventory can be configured to report unencrypted objects, and S3 Batch Operations allows you to copy each object over itselfapplying encryption in the process. This solution scales well even with millions of objects.

Why the other options are incorrect:

Creating a new S3 bucket and manually downloading and re-uploading files introduces significant complexity and risk, especially with millions of objects. It requires managing temporary storage, scripting, and may result in longer downtime or inconsistent data.

Enabling SSE-KMS with a new KMS key and turning on versioning only affects new uploads and new versions of objects. It does not retroactively encrypt existing unencrypted objects. It also adds complexity with KMS permissions and potential cost considerations.

Manually browsing and modifying unencrypted objects in the AWS Management Console is extremely inefficient for large datasets. This method does not scale and is not practical when dealing with millions of files.

Why this question is important: This scenario evaluates your understanding of S3 encryption options and your ability to handle large-scale data operations efficiently using AWS-native tools. It highlights the importance of automation and using scalable services like Inventory and Batch Operations.

Tip: If you see a need to apply encryption to existing S3 objects, look for copy-based solutions (like S3 Batch Operations). Avoid manual steps or solutions that dont operate at scale.

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