Which combination of actions will meet these requirements?

Explanations:

Auto Discovery is a feature of Amazon ElastiCache that allows clients to discover the endpoints of nodes dynamically. While it is beneficial for scalability and managing cluster changes, it does not directly relate to implementing a high-availability caching service.

Amazon ElastiCache for Memcached can provide caching capabilities but lacks support for advanced features like persistence and replication, which are essential for high availability. Therefore, it is not the best choice for ensuring high availability in this scenario.

Amazon ElastiCache for Redis offers in-memory data storage with advanced features like replication, persistence, and automatic failover. These features contribute to high availability, making it a suitable option for caching while ensuring system resilience.

Enabling Multi-AZ for the data store provides high availability by automatically replicating data across multiple Availability Zones. This setup minimizes downtime and ensures data durability, making it a critical action to implement alongside the caching service.

Multi-threading is a performance feature that may improve throughput but does not directly impact high availability. It is not relevant in the context of ensuring that the caching service remains operational and resilient.