Which General Order directs a sentry to maintain military posture while on duty?

The 2nd General Order is the directive that instructs a sentry to "walk my post in a military manner, keeping always on the alert and observing everything that takes place within sight or hearing." This order emphasizes maintaining a professional appearance and demeanor, which embodies the concept of military posture. It requires the sentry to perform their duties with a sense of discipline and focus, ensuring they are always attentive to their surroundings while adhering to standard military conduct.

In contrast, the other General Orders focus on different aspects of a sentry's duties, such as addressing the security of the post, reporting violations, and adhering to orders from superiors, which do not specifically reference the requirement to maintain military posture while on duty. Therefore, the 2nd General Order is the correct choice as it encapsulates the essence of how a sentry should carry themselves while performing their duties.