A company stores millions of frequently accessed objects in Amazon S3 Standard with server-side encryption using AWS KMS (SSE-KMS). As access grows, KMS costs on the monthly bill are rising due to the high request volume. The solutions architect needs to reduce costs with minimal application changes and the least operational overhead. Which solution should be used?

The correct solution is to use SSE-S3. SSE-S3 encrypts objects with keys managed by Amazon S3, eliminating KMS request charges, which directly addresses the rising KMS costs. S3 Batch Operations provides an efficient way to copy millions of existing objects with minimal operational overhead and no application changes. Using SSE-C would require the company to manage encryption keys, increasing operational overhead. While SSE-C avoids KMS costs, it shifts the burden of key management to the customer. Using AWS CloudHSM would significantly increase complexity and cost, as CloudHSM is a dedicated hardware security module. It's an overkill for simply reducing KMS request costs and introduces substantial operational overhead. Switching to S3 Intelligent-Tiering and archiving objects to S3 Glacier Deep Archive focuses on storage cost optimization based on access patterns, not on reducing KMS request costs for frequently accessed objects. The problem states the objects are "frequently accessed," making archiving an unsuitable solution for the core issue.