AAn object storage service for storing and retrieving any amount of data
BA block storage service for attaching persistent volumes to EC2 instances
CA file storage service that many EC2 instances can mount at the same time
DA relational database service for running queries with standard SQL syntax
Why this is the answer
Amazon S3 (Simple Storage Service) stores data as objects in buckets, accessed over HTTP APIs, with virtually unlimited capacity. Block volumes for one instance are EBS, a shared file system is EFS, and a managed SQL database is RDS.