Which command verifies the /etc/fstab configuration file before rebooting?

The command that verifies the /etc/fstab configuration file before rebooting is mount -a. This command is specifically used to mount all filesystems listed in the /etc/fstab file that are not currently mounted. When executed, it checks the syntax and the overall correctness of the entries in the fstab configuration. If there are issues with the configuration file, mount -a will report errors without applying any changes, making it an effective way to validate the configuration before a system reboot.

The other commands provided do not serve this specific purpose. For instance, swapon -s is used to display active swap space information but does not interact with the fstab file directly. systemctl list-unit-files lists systemd unit files and their states but does not check mount points or fstab entries. Lastly, gdisk is a tool for partitioning disks and does not perform any checks related to fstab or file systems mounting.