Final answer:
The correct header for SSE-S3 encryption in Amazon S3 is 'x-amz-server-side-encryption'. This header signals Amazon S3 to use server-side encryption with Amazon-managed keys.
Step-by-step explanation:
When using Amazon S3 and you wish to activate server-side encryption with Amazon S3-managed keys (SSE-S3), you need to include a specific header in your request. The correct header is B) x-amz-server-side-encryption. When this header is set to AES256, Amazon S3 manages the encryption as it writes to disks, and it manages decryption when you access your objects.