Which solution will meet these requirements MOST cost-effectively?

Explanations:

Reducing the RDS DB instance size will not improve performance. Changing to Magnetic storage decreases performance and increases latency, which is not suitable for processing BLOBs efficiently. Although it reduces costs, it compromises availability and resilience.

While increasing the RDS DB instance size and switching to Provisioned IOPS may improve performance, this solution is likely to be more expensive than necessary. Increasing storage to 24 TiB might not address the core issue of handling BLOBs effectively and could lead to higher costs without solving the performance problem.

Storing documents in Amazon S3 is a cost-effective solution that enhances performance and scalability. S3 is designed for high availability and durability, making it suitable for storing large documents. Keeping metadata in RDS reduces the load on the database while maintaining relational integrity. This option provides a resilient architecture and optimizes costs.

Migrating to DynamoDB might provide scalability benefits, but it involves significant changes to the application and data model. Using AWS DMS would also add complexity and potential downtime. Additionally, DynamoDB is not optimized for BLOB storage, which could lead to performance issues. This solution may not be as cost-effective as storing documents in S3.