What action must be taken to extinguish the ACK GUARD DISCONNECT message?

To extinguish the ACK GUARD DISCONNECT message, pressing the autothrottle disconnect button on the SDU (System Display Unit) is the correct action. This specific button is designed to acknowledge and clear any alert or warning messages related to the autothrottle system, indicating that the system is no longer in a state of alert.

This action directly communicates with the avionics to reset the alert status, thereby addressing the underlying condition that triggered the ACK GUARD DISCONNECT message. It is a procedural step that allows the pilot to regain normal system operation.

Other options might seem related to managing engine or throttle controls, but they do not specifically serve the purpose of directly addressing the alert associated with the autothrottle system. Therefore, using the autothrottle disconnect button is the most effective and appropriate response in this scenario.