An application will store personal health information (PHI) and must keep the data encrypted at all times. Data is stored in an encrypted Amazon RDS for MySQL DB instance. The developer wants to improve performance by caching frequently accessed data and needs the ability to sort or rank cached datasets. Which solution meets these requirements?

The correct solution is to launch an Amazon ElastiCache for Redis cluster with encryption in transit and at rest. Redis supports sorting and ranking of datasets, which is a specific requirement. It also offers encryption features crucial for PHI. ElastiCache for Memcached does not inherently support sorting or ranking of datasets, making it unsuitable for this requirement. An Amazon RDS for MySQL read replica would improve read performance but does not function as a dedicated caching layer for frequently accessed data in the same way ElastiCache does, nor does it provide the sorting/ranking capabilities for cached data. Amazon DynamoDB with DAX is a NoSQL solution designed for high-performance key-value and document data. While DAX provides caching, the primary data store is DynamoDB, which is not ideal for caching frequently accessed data from an existing RDS MySQL instance, and it doesn't directly offer the required sorting/ranking capabilities for cached data in the same manner as Redis.