What must be done in terms of the waste system during No.2's checks?

The correct choice highlights the importance of checking both the waste and water systems together during No.2's checks. This approach is significant because it allows for a comprehensive assessment of critical systems that can impact cabin service and hygiene. Both the waste and water systems are interconnected; for example, a malfunction in one system can affect the other. Ensuring both systems are operating properly helps maintain safety and comfort for passengers and crew. This holistic check reinforces best practices in resource management, promoting efficiency, and minimizing risks related to sanitation and maintenance issues on board.

In contrast, focusing solely on the waste quantity or ensuring it is empty, while essential tasks, does not address the complete operational status of both systems. Similarly, while securing the waste system is important, it does not encompass the essential checks needed for safe and effective cabin operations as highlighted by the correct answer.