What effect does the command systemctl enable have on designated units?

The command systemctl enable is used to configure system services (or units) to start automatically during the system boot process. When you run this command followed by a unit name, it creates the necessary symbolic links in the system's target directories to ensure that the designated unit is activated whenever the system boots up.

This functionality is crucial for services that need to be up and running as soon as the system initializes. For instance, if a service provides essential functionality like a web server or a database, enabling it ensures that users can access these services without manual intervention after rebooting the system.

In contrast, the other options relate to disabling or stopping services, which does not align with the purpose of the systemctl enable command. The command does not prevent starting, disable the service, or stop it immediately; its sole function is to prepare the service to start during the boot sequence.