What feature of EBS allows a single volume to be attached to multiple EC2 instances simultaneously?

EBS Multi-Attach is a feature that allows a single Elastic Block Store (EBS) volume to be attached to multiple EC2 instances at the same time. This capability is particularly useful for applications that require high availability and redundancy, as it enables different EC2 instances to access the same volume concurrently.

When using EBS Multi-Attach, it's important to note that the attached instances must be part of an Amazon Elastic Compute Cloud (EC2) cluster, and the file system used on the volume must be cluster-aware (such as a multi-attach-enabled file system). This allows for shared data storage across instances, supporting use cases such as clustered applications, databases requiring high availability, or shared file systems.

The other features mentioned, such as EBS Encryption, relate to securing data at rest and in transit, EBS Snapshot deals with creating backups of EBS volumes, and EBS Shared Access is not a recognized feature within the context of AWS EBS services. Thus, EBS Multi-Attach is the only correct answer related to the ability to attach a single volume to multiple EC2 instances simultaneously.