A user wants to back up a device's OS and data.

A system image is a complete snapshot of everything on a device's storage at a given point in time, including the operating system, installed programs, system settings, and all user files. This method is the best way to back up the OS and data comprehensively because it allows for the restoration of a system to its exact state at the time the image was taken. This is particularly useful in disaster recovery scenarios where it's crucial to restore a system quickly and efficiently to minimize downtime.