Which data-at-rest encryption method encrypts all the data stored on the volume and all snapshots created from the volume?

The rationale behind identifying whole instance encryption as the correct option comes from its comprehensive approach to data protection. Whole instance encryption secures not just files but the entire virtual machine, which includes all data stored on the volume and any snapshots created from it. This layer of security ensures that, from the moment the instance operates, everything is encrypted, providing a robust defense against unauthorized access.

In contrast, the other methods focus on more specific or granular levels of encryption. Volume encryption primarily targets individual storage volumes rather than the entire instance. While it does protect data at rest, it does not extend to the full breadth of what whole instance encryption offers. Directory encryption is limited to protecting specific directories, leaving other portions of the instance or volume unprotected. Block encryption operates at the level of individual blocks of data rather than encompassing the entire instance or volume, potentially missing some critical data during encryption.

Thus, whole instance encryption serves as an all-encompassing solution that secures all aspects of data at rest, making it the optimal choice for a method that encrypts all data stored on the volume along with any created snapshots.