The correct answer is B, Amazon ElastiCache. Amazon ElastiCache is a service that simplifies the deployment, operation, and scaling of in-memory data stores or caches in the cloud. It supports two open-source in-memory data stores: Memcached and Redis. In contrast, Amazon DynamoDB is a NoSQL database service that provides fast and predictable performance with seamless scalability, Amazon RDS is a relational database service that makes it easier to set up, operate, and scale a relational database in the cloud, and Amazon Timestream is a time-series database service optimized for handling time-series data. Therefore, for in-memory data storage, Amazon ElastiCache is the appropriate choice.