Which of these commands is used to configure virtual terminal lines on a router?

The command used to configure virtual terminal (VTY) lines on a router is "line vty 0 4." This command is specifically designed to access and configure the virtual terminal lines, which are used for remote access to the router via protocols such as Telnet or SSH.

When you specify "line vty 0 4," you are selecting the VTY lines 0 through 4, allowing configurations for these five simultaneous connections. This capability is crucial for managing routers in environments where multiple administrators might need to access the router concurrently.

In contrast, the command that defines "line console 0" is used to configure the console line, which is the physical connection for direct device access, making it unrelated to remote access configuration. The "interface vty 0" command is incorrect as it does not follow the syntax used in Cisco IOS; interface configurations do not apply to VTY lines directly. Lastly, the "line aux 0" command is meant for auxiliary lines, which support a modem connection and are also not relevant to VTY configurations.

Thus, "line vty 0 4" directly addresses configuring remote access capabilities on the router, making it the correct command for this purpose.