What type of encryption solution should be used to protect files both in motion and at rest, with granular per-user security?

The correct choice is file encryption, as it specifically addresses the need for granular per-user security while protecting files both in motion and at rest. File encryption allows individual files to be encrypted, often with unique encryption keys for each user. This level of customization ensures that access to the files can be tightly controlled based on user permissions, allowing for a more secure environment when multiple users need to interact with sensitive data.

In contrast, disk encryption protects entire drives or volumes, which does not provide the same granularity in access control across individual files. While disk encryption secures data at rest, it typically affords less flexibility in terms of user-specific security measures.

Database encryption secures information stored within databases, yet it mainly focuses on database access rather than the broader file level, limiting its effectiveness in scenarios where individual file access needs distinct control.

Network encryption secures data while it is being transmitted over a network but does not apply to files stored at rest. Therefore, it does not meet the dual requirement of protecting files both in motion and at rest with specific user-level controls.

File encryption is thus the most suitable choice as it combines protection for data at rest and in transit with the ability to enforce specific access rights for different users.