Which solution will meet these requirements with the LEAST operational overhead?

Explanations:

Amazon DynamoDB Accelerator (DAX) is a fully managed, in-memory caching service for DynamoDB that can significantly improve response times from milliseconds to microseconds. It is designed to integrate seamlessly with DynamoDB, reducing operational overhead by automatically handling cache management and invalidation.

Migrating to Amazon Redshift would not be appropriate as it is primarily a data warehousing service designed for analytical workloads, not for the low-latency transactional operations typically required in serverless applications. Additionally, this option would involve significant operational overhead for migration and management.

Amazon RDS is a managed relational database service that provides benefits like automated backups and scaling. However, it would not inherently provide the microsecond response times needed for this application. It also introduces additional operational complexity compared to using DynamoDB with DAX.

Amazon ElastiCache for Redis is a caching solution that could improve performance; however, it requires additional management and operational overhead to set up and maintain a separate cache layer. It does not integrate directly with DynamoDB as seamlessly as DAX does.