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.