The solution requires a shared storage layer accessible by multiple pods across different Availability Zones, optimized for many small files, and integrated with a backup solution. Amazon EFS is a fully managed, elastic NFS file system designed for this exact use case. It provides POSIX-compliant file access, can be mounted by multiple pods simultaneously across AZs, and offers high performance for metadata-intensive workloads (many small files). Integrating EFS with AWS Backup allows for automated, policy-based backups with the required 1-year retention, meeting all constraints of the question.

B: Amazon EBS Multi-Attach volumes can only be attached to instances within a single Availability Zone, which fails the multi-AZ requirement of the EKS cluster. C: Amazon S3 is an object store, not a POSIX-compliant file system. Mounting it requires an adapter (like s3fs-fuse), which adds latency and is not performant for small file I/O. D: Using local pod storage fails the primary requirement for files to be shared and accessible across all application instances, as this storage is ephemeral and isolated to each pod.

1. Amazon EKS User Guide - Amazon EFS CSI driver: "Amazon EFS provides a scalable, elastic, cloud-native, and highly available shared file system that can be shared by pods running in any Availability Zone in your cluster." This confirms EFS is the standard for multi-AZ shared storage in EKS.

2. Amazon EBS User Guide - Attach your volume to multiple instances with Amazon EBS Multi-Attach: "You can attach a Multi-Attach enabled volume to up to 16 Nitro-based instances in the same Availability Zone." This document explicitly states the single-AZ limitation.

3. AWS Storage Blog - Choosing the right storage for your container strategy: "Amazon EFS is ideal for containerized applications that need shared file storage with high levels of throughput and IOPS for workloads like web serving, content management, and data analytics." This highlights EFS's suitability for the described workload.

4. AWS Backup Developer Guide - Feature availability by resource: The "Supported AWS resources" table lists Amazon EFS as a supported service for backup, restore, and cross-region/account copy, confirming its compatibility with the backup requirement.

5. Amazon EFS User Guide - Performance for Amazon EFS: "For file system operations, Amazon EFS is a low-latency file system. File system operations are typically in the low, single-digit milliseconds." This supports the claim of high performance for file-based workloads.