Refer to the exhibit.

The correct answer is C. no username and only the password " cisco123 " .

aaa authentication login group1 group radius line

This means:

Try RADIUS authentication first

If RADIUS fails/unreachable → fallback to line authentication

line con 0

password 0 cisco123

login authentication group1

Because RADIUS servers are not reachable, the router falls back to: → line authentication

When using line authentication, the router uses:

The configured line password

No username is required

Thus, login will prompt only for a password:

Password: cisco123

A. username " cisco " password " cisco " This uses the local user database, but local is NOT configured as fallback in group1.

B. password " test123 " This password is configured under VTY lines, not the console.

D. username " cisco " password " cisco123 " Combines local username with console password, but AAA fallback is to line, not local.

AAA authentication order matters:

group radius line → fallback is line password

group radius local → fallback is local username/password

Console + RADIUS down → uses line password only