The correct answer is Amazon ElastiCache. This service provides an in-memory data store that supports Memcached and Redis cache engines, which are ideal for use cases requiring extremely low-latency data access, such as real-time personalization of content. It is optimized to deliver high-performance, in-memory caching. Amazon RDS is geared toward traditional relational databases and would provide higher latency in comparison to an in-memory cache. Amazon DynamoDB Accelerator (DAX) is a caching solution but it is specific to augmenting access to DynamoDB tables, not a standalone in-memory cache for varied backend sources. Amazon Neptune is tailored for graph databases and does not specialize in low-latency caching. AWS Glue is an ETL service with no built-in caching capability. Lastly, Amazon Quantum Ledger Database (QLDB) is used for ledger-based applications and does not function as an in-memory cache.