What command is used to ensure a designated unit configuration changes without stopping the service?

The command that is used to ensure that configuration changes for a designated unit take effect without stopping the service is "systemctl reload." When you execute this command, it sends a signal to the active service, instructing it to reload its configuration files and apply any changes made, while the service continues to run. This is particularly useful for services that need to have their configurations refreshed without incurring downtime, thereby maintaining availability.

Other commands listed do not serve the purpose of reloading configuration. For example, an "apply" command does not exist in systemctl's context, and the command "update" is not associated with reloading configurations. Similarly, "modify" is not a recognized option in the systemctl command set for handling running services. Therefore, when needing to implement changes in configuration seamlessly, using "systemctl reload" is the appropriate approach.