Which action aligns with entering "Router(config-if)#" in command-line interface?

Entering "Router(config-if)#" in the command-line interface indicates that the user has successfully accessed the interface configuration mode of a router. This mode allows network administrators to make specific changes and configurations related to a particular interface on the router, such as setting IP addresses, configuring access control lists, or enabling and disabling the interface.

In this context, once you see the "Router(config-if)#" prompt, it signifies that you are not in global configuration mode anymore but have drilled down to focus on the settings for a specific interface, such as an Ethernet or Serial interface. This is critical for precise control over network settings and ensures that configurations apply only to the intended interface rather than affecting the entire device.

The other options do not align with the context of that prompt. Exiting configuration mode would lead you back to a higher privilege level, while configuring the router hostname or invoking routing protocols typically occurs in different modes higher up in the configuration hierarchy.