According to the ninth order, what should the sentry do?

In the context of the ninth order of a sentry, the primary responsibility is to ensure that any situation that arises, which is not explicitly covered by existing instructions, is referred to a higher authority. This means that if a sentry encounters something unusual or requires guidance in handling a specific scenario, they must promptly notify the officer of the deck. This principle is vital for maintaining security and order on a vessel.

This action underscores the importance of the chain of command and the necessity for sentries to rely on established leadership when faced with ambiguity. By contacting the officer of the deck, the sentry ensures that decisions are made with proper authority and expertise, rather than risking mistakes by acting independently without guidance.

While the other choices involve important responsibilities—reporting incidents, tracking personnel movements, and maintaining communication with other posts—they do not accurately reflect the specific directive associated with the ninth order, which emphasizes the necessity of escalating situations outside the sentry's direct instructions to the appropriate supervisory figure.