Answer :
The command to use when specifying that a local username should be used for authentication on the VTY lines is 'login local'. This tells the Cisco device to verify users against its local database when accessing via telnet or SSH. So, the correct option is A) login local.
The correct command to specify that a local username of 'admin' from the running configuration should be used for authentication on the VTY lines is login local. This command is used in Cisco devices to tell the device to use the local database for authenticating users trying to access the device via the VTY lines (telnet or SSH). The local database consists of usernames and passwords defined on the device itself. Therefore, when the 'login local' command is configured under the VTY lines, any user trying to connect must provide credentials that match those in the local database.
It's important not to confuse the 'login local' command with other commands that sound similar. For instance, 'login username admin' is not a correct command, and 'login authentication local' is used in conjunction with AAA (Authentication, Authorization, and Accounting) to refer to a defined method list, not the local user database directly. Lastly, 'login admin local' is not a valid Cisco command. So, the correct option is A) login local.
