Amazon DynamoDB DAX is the correct choice for adding an in memory cache to DynamoDB because it is purpose built to provide microsecond read latency and it integrates with DynamoDB so the application data model and client code do not need to change.

Amazon DynamoDB DAX is a fully managed, in memory cache that sits in front of DynamoDB and it speaks the same DynamoDB API so reads can be served from the cache without rewriting access logic. This reduces DynamoDB read capacity usage and improves response times for repeated reads while keeping the data model intact.

Amazon ElastiCache is a general purpose in memory caching service that supports Redis and Memcached, but it is not a drop in cache for DynamoDB and using it typically requires application changes to implement caching logic, so it does not meet the requirement of avoiding code or data model changes.

AWS Key Management Service (KMS) handles encryption key management and it provides no caching capabilities, so it cannot be used to speed up reads for DynamoDB.

Amazon Simple Queue Service (SQS) is a message queuing service for decoupling components and it does not cache data or reduce read latency for DynamoDB queries.

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