A user needs a relational database but cannot manage the hardware, resiliency, or replication. Which AWS service should they choose?

Amazon RDS (Relational Database Service) is the correct choice because it is a managed service that handles the operational heavy lifting of a relational database, including hardware provisioning, patching, backups, and replication. This directly addresses the user's need to avoid managing hardware, resiliency, or replication. Running MySQL on Amazon ECS or Amazon EC2 would require the user to manage the underlying infrastructure, operating system, and database software themselves, which contradicts their requirement. Amazon ElastiCache for Redis is an in-memory data store, not a relational database, and therefore does not meet the core requirement for a relational database.