What command is used to exit interface configuration mode on a router?

The command used to exit interface configuration mode on a router is "exit." This command is fundamental in Cisco IOS, enabling users to navigate back to the previous configuration level. When in interface configuration mode, issuing the "exit" command will take the user back to global configuration mode. This command effectively manages the hierarchy of configurations in the Cisco IOS, allowing for streamlined navigation between the different modes of configuration.

The other options provided, while they seem plausible, are not valid commands within Cisco IOS. "Quit," "leave," and "bye" do not perform the same function and will not successfully exit the interface configuration mode. In Cisco's command-line environment, it is essential to use the correct terminology to ensure that commands are recognized and executed properly. Thus, using "exit" is the only correct method to achieve this specific task.