Which solution will meet these requirements with the LEAST complexity?

Explanations:

Amazon ElastiCache for Memcached is a fully managed, in-memory caching service that can help offload read requests from the main database. It is suitable for scenarios where applications need to retrieve frequently accessed data quickly, thus improving performance with minimal complexity in setup and management.

Replicating data to Amazon DynamoDB and setting up a DynamoDB Accelerator (DAX) cluster adds unnecessary complexity to the solution. This involves managing a different database service (DynamoDB) and configuring DAX for caching, which may not directly address the caching needs for the existing application that relies on RDS.

Configuring Multi-AZ deployments with a standby instance primarily focuses on high availability and disaster recovery, not performance improvement. While it does allow for some read scaling, it does not effectively offload read traffic during peak loads and may not solve the query speed issues.

Amazon ElastiCache for Redis can also be used to offload read requests, but it is generally considered more complex than Memcached due to its additional features (like data persistence and advanced data structures). In this context, Memcached may be a simpler solution to implement for caching frequently accessed data.