Explanation

Amazon ElastiCache is the correct choice because:

• In-memory data store: ElastiCache provides fully managed in-memory data stores using Redis or Memcached
• Open source compatibility: It supports popular open-source in-memory caching engines (Redis and Memcached)
• Managed service: AWS handles the operational overhead while maintaining compatibility with open-source APIs

Why other options are incorrect:

• Amazon DynamoDB: NoSQL database service, not primarily an in-memory data store
• Amazon Elastic Block Store (EBS): Block-level storage volumes for EC2 instances, not an in-memory data store
• Amazon Redshift: Data warehousing service for analytics, not an in-memory caching solution

ElastiCache is specifically designed for use cases requiring high-performance, low-latency in-memory caching with open-source compatibility.