Which situation must be reported according to the Eighth Order of the Sentry?

The correct choice focuses on the requirement to report any incidents of disorder as stated in the Eighth Order of the Sentry. This order emphasizes the importance of maintaining vigilance and discipline within the post. Reporting incidents such as disturbances, unauthorized personnel, or any irregular activities is crucial for maintaining security and order.

In a military context, being alert to potential threats and disturbances is a fundamental duty. Incidents of disorder can indicate a breach of security or other serious issues that need immediate attention. By ensuring that these incidents are reported, the overall safety and effectiveness of the sentry’s post are upheld.

In contrast, personal grievances, while they may affect a soldier's morale or mental state, do not pertain directly to the operational responsibilities of a sentry. Changes in the guard schedule are typically managed through specific protocols rather than being reported as an incident of disorder. Ceremonial events, while important, do not require reporting in the context defined by the Eighth Order. Thus, the focus on reporting incidents of disorder aligns directly with the responsibilities of a sentry in maintaining security.