The correct answer is D. NTP trusted-key 1.
ntp authenticate
ntp authentication-key 1 md5 NTP-p455w0rd
To fully enable NTP authentication, three components are required:
* Enable authentication globally
ntp authenticate
* Define the authentication key
ntp authentication-key < key-id > md5 < password >
* Trust the key
ntp trusted-key < key-id >
* The configuration already defines key 1
* The missing step is to mark this key as trusted
* Without ntp trusted-key 1 , the router will not use the key, even if it is defined
* A. NTP server 10.1.1.1 key NTP-p455w0rd Incorrect syntax. The key must be referenced by key ID (number), not password.
* B. NTP trusted-key NTP-p455w0rd Incorrect. Trusted-key uses the key ID, not the password.
* C. NTP server 10.1.1.1 key 1 This command is used on a client, not required to complete authentication on the server itself in this context.
For NTP authentication, always remember:
authenticate # authentication-key # trusted-key
If any of these three are missing, authentication will not work properly.