What must be ensured about the fuel systems in standard classification aircraft?

In standard classification aircraft, having a positive method to shut off fuel is crucial for safety and operational control. This ensures that in the event of an emergency or if a malfunction occurs, the fuel supply can be effectively stopped to prevent potential hazards such as fuel leaks or fires. A positive shutoff mechanism provides pilots with the ability to manage fuel flow actively, which is essential during emergency situations or abnormal operations.

The requirement for a dependable fuel shutoff mechanism aligns with aviation safety standards that prioritize risk mitigation. It also reflects best practices in aircraft design, ensuring that fuel systems can be controlled with certainty and reliability.

As for the other options, while they pertain to aspects of fuel management or efficiency, they do not specifically address the critical safety need for a definitive means of shutting off fuel. This highlights why option D is the most appropriate answer, underscoring an essential safety feature in aircraft fuel systems.