If PPP authentication fails, which type of packet will be sent to the authenticated peer...

In PPP (Point-to-Point Protocol) authentication, if the authentication fails, the authenticator sends an Authenticate-Reject message.

Option A (Incorrect - Authenticate-Ack): Sent when authentication is successful.

Option B (Incorrect - Authenticate-Reply): Not a valid PPP authentication message.

Option C (Correct - Authenticate-Reject): Sent when authentication fails, and the connection is rejected.

Option D (Incorrect - Authenticate-Nak): Used for PPP negotiation, not authentication failure.

Thus, the correct answer is C.