A fleet of EC2 instances across multiple AZs must read and write to the same set of files concurrently, with the storage automatically scaling. Which service fits?
A) A single EBS volume attached to all instances.
B) Amazon EFS (Elastic File System).
C) Instance store on each instance.
D) An S3 bucket mounted as a block device.
Correct Answer: B
Explanation: Amazon EFS is a managed, elastic NFS file system mountable concurrently by many instances across multiple AZs, scaling automatically. (A) a standard EBS volume attaches to one instance at a time and is AZ-bound. (C) instance store is local and ephemeral. (D) S3 is object storage, not a POSIX block/file device.