Which solution meets these requirements and is MOST cost-effective?

Explanations:

Amazon EBS snapshots are not designed for sharing data across multiple instances in real-time. They are primarily for backup and recovery purposes, and sharing a snapshot would require creating volumes that cannot be easily synchronized or accessed concurrently by multiple instances.

While creating an S3 bucket with a lifecycle policy to transition data to S3 Standard-IA is cost-effective for infrequently accessed data, it does not provide a file system interface for concurrent access by multiple users. S3 is object storage and not suitable for scenarios requiring file system semantics with multiple concurrent users.

Amazon EFS is suitable for shared access across multiple instances, but setting the throughput mode to Provisioned could lead to higher costs. It is more cost-effective to utilize EFS with lifecycle policies that manage infrequent access data instead of a provisioned throughput mode, especially since the majority of data is accessed rarely.

Creating an Amazon EFS file system with a lifecycle policy to transition infrequent access data to EFS IA is the most cost-effective solution. EFS provides a shared file system interface suitable for concurrent access, and using EFS IA optimizes costs for data that is accessed rarely while ensuring synchronization across multiple instances.