Which of the following actions must a sentry take if they observe a fire?

The correct action for a sentry to take upon observing a fire is to give the alarm. This response is critical for ensuring the safety of all personnel and for initiating emergency procedures. By sounding the alarm, the sentry alerts others in the vicinity about the immediate danger, enabling swift evacuation and response measures, such as calling the fire department or initiating fire control protocols.

Prompt action in the event of a fire can save lives and minimize damage to property. It is crucial for any individual in a security or safety role to understand that their priority is to communicate any hazards to the appropriate people or services without delay.

Other options, such as talking to no one about it, would hinder emergency response efforts and could put lives at risk. Calling the petty officer of the watch can be important, but it should not be the first action taken when a fire is visible; alerting everyone around is more immediate. Ignoring the fire unless instructed otherwise is dangerous, as it compromises safety and only delays necessary action that could prevent a worsening situation.