To stop and simultaneously restart designated units, which command should be used?

The command that effectively stops a designated service and immediately restarts it is "systemctl restart." When executed, this command sends a stop command to the specified service and then follows it with a start command, all in one action. This is particularly useful for applying changes made to a service's configuration without needing to stop it first and then start it again in two separate steps.

In contrast, other options like "systemctl enable" are used to enable a service to start automatically at boot, while "systemctl stop" and "systemctl start" are used individually to stop or start services, respectively, which does not achieve the simultaneous nature of stopping and starting in one command. Thus, "systemctl restart" is the most efficient choice for stopping and simultaneously restarting designated services.