The correct configuration to accomplish the task of using TACACS for authentication and allowing login without any credentials if TACACS is unavailable is option C. This configuration uses the aaa new-model command to enable AAA (Authentication, Authorization, and Accounting), and the aaa authentication login default group tacacs+ none command specifies that the default login authentication method should use TACACS+ first (group tacacs+) and if TACACS+ is not available, then no authentication will be required (none). The line vty 0 4 command applies this authentication method to the VTY lines, which are used for remote access to the router.
According to the requirements (first use TACACS+, then allow login with no authentication), we have to use "aaa authentication login ...group tacacs+ none" for AAA command.
The next thing to check is the if the "aaa authentication logindefault" or "aaa authentication loginlist-name" is used. The 'default' keyword means we want to apply for all login connections (such as tty, vty, console and aux). If we use this keyword, we don't need to configure anything else under tty, vty and aux lines. If we don't use this keyword then we have to specify which line(s) we want to apply the authentication feature.
From above information, we can find out answer 'R1#sh run | include aaa aaa new-model aaa authentication login default group tacacs+ none aaa session-id common R1#sh run | section vty line vty 0 4 password 7 0202039485748 If you want to learn more about AAA configuration, please read ourAAA TACACS+ and RADIUS Tutorial - Part 2.
For your information, answer 'R1#sh run | include aaa
aaa new-model
aaa authentication login telnet group tacacs+ none
aaa session-id common
R1#sh run | section vty
line vty 0 4
R1#sh run | include username
R1#' would be correct if we add the following command under vty line ("line vty 0 4"): "login authentication telnet" ("telnet" is the name of the AAA list above)