What should a sentry do if they observe a fire or disorder?

The correct choice emphasizes the critical importance of prompt action in the event of a fire or disorder. A sentry who observes such incidents must immediately give the alarm to ensure the safety of personnel and the secure management of the situation. This action is crucial because a delayed response could lead to greater risks, including injury to individuals or escalating the emergency.

The act of giving the alarm alerts those in command and initiates an immediate response, allowing for the deployment of personnel and resources to address the situation effectively. By prioritizing this action, the sentry plays a vital role in maintaining safety and order.

While other responses, such as calling the officer of the deck or reporting to the commanding officer, are valid, they are secondary actions that rely on the initial step of sounding an alarm. Ignoring the situation is never appropriate, as it can lead to devastating consequences. Thus, taking quick action by giving the alarm is the most responsible and required course of action for a sentry in this type of circumstance.