Which command is used to reset a device to factory defaults on a Cisco router?

The command used to reset a Cisco router to its factory defaults is "erase startup-config." This command removes the startup configuration file that is stored in the NVRAM (Non-Volatile Random Access Memory) of the router. By erasing this file, the router loses all the configurations that were applied, essentially returning it to a state similar to when it was first deployed—that is, without any user-defined settings.

After executing this command, typically, the configuration will need to be reloaded or the router will need to be rebooted for the changes to take effect. Upon reboot, the router will run with the default settings as no configuration will be present in the startup file. This process is essential when performing maintenance, troubleshooting, or when one wants to make a fresh start with the device.

While other options might seem relevant, they do not serve this particular function. For instance, "delete config" and "reset config" do not correspond to recognized Cisco commands in the context of resetting factory settings. The "factory reset" terminology may be seen in other devices or contexts but is not an official command on Cisco routers. Hence, "erase startup-config" is the correct command for achieving a factory reset on Cisco routers.