Which of the following is true when a file system, which is neither listed in /etc/fstab nor known to the system, is mounted manually?

When a file system that is neither listed in /etc/fstab nor recognized by the system is mounted manually, the correct statement regarding systemd's behavior is that the command systemctl mountsync can be used to create a mount unit based on the existing mount. This is significant because systemd has a component known as "mount units," which are systemd's way of managing file system mounts. When a file system is mounted manually, systemd does not automatically track this mount unless it is informed about it via a mount unit.

Using the systemctl mountsync command allows you to effectively register the manually mounted file system with systemd. This creates a persistent and more managed approach to handling the mount, enabling monitoring and ensuring that the mount remains appropriately managed by systemd beyond the simple manual action.

This contrasts with other behaviors of systemd when a file system is mounted without an associated unit. Without a mount unit created, the manual mount may not have lifecycle management, including automatic unmounting if systemd doesn’t recognize it as a service to manage.

The other choices suggest different behaviors that do not accurately reflect how systemd interacts with manually managed mounts. Understanding the implication of mount units and how they are created is crucial for efficiently working with systemd