Which solution meets these requirements?

Explanations:

Amazon Elastic File System (EFS) is designed for file storage that is accessible concurrently by multiple EC2 instances. It provides scalability, high availability, and durability, making it ideal for storing core company assets like client case files. EFS also automatically scales as the number of files grows, ensuring redundancy and reliability.

Amazon Elastic Block Store (EBS) provides block storage for individual EC2 instances and is not designed for concurrent access by multiple instances. While EBS is durable and provides redundancy within a single Availability Zone, it does not support the simultaneous access requirement specified in the scenario.

Amazon S3 Glacier Deep Archive is designed for long-term archival storage and is not suitable for frequently accessed files. It has a retrieval time of hours, making it impractical for scenarios requiring immediate access. While S3 offers redundancy and durability, Glacier Deep Archive does not meet the need for simultaneous access by multiple application servers.

AWS Backup is a service for centralized backup management, not a storage solution. It can help automate and manage backups of various AWS resources, but it does not provide a means to store and access client case files directly. It does not fulfill the requirement for redundancy in the storage solution itself.