The correct answer is: Modify the S3 bucket policy to deny any PutObject request that does not include the x-amz-server-side-encryption header.

To enforce encryption on all new objects uploaded to an Amazon S3 bucket, the best practice is to use a bucket policy that explicitly denies any PutObject request missing the x-amz-server-side-encryption header. This ensures that clients must specify server-side encryption (such as SSE-S3 or SSE-KMS) when uploading objects, thus enforcing compliance with data protection policies.

The option Change the bucket policy to reject PutObject requests that lack the s3:x-amz-acl header set to private controls access permissions, not encryption. It ensures that uploaded objects are private, but it does not enforce any form of encryption.

The option Configure the S3 bucket policy to block any uploads without aws:SecureTransport set to true in the request is focused on enforcing HTTPS for secure transmission, which protects data in transit but does not enforce encryption at rest.

The option Set the S3 bucket policy to deny any PutObject request that does not include the s3:x-amz-acl header again relates to access control, not encryption. While it can enforce that the object has an ACL, it has no bearing on whether the object is encrypted.

This question is important because it tests your knowledge of S3 bucket policies, security best practices, and how to enforce compliance for data stored in AWS. When encryption is required, always focus on headers that dictate how the object is storednot how it is transmitted or shared.

A useful trick is to look for the presence of x-amz-server-side-encryption when thinking about encryption enforcement. AWS provides this header specifically for enforcing SSE-S3 or SSE-KMS encryption at upload time.

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