When must the cross-feed operation be turned off?

The correct choice regarding when the cross-feed operation must be turned off is specifically during takeoff and landing. This is crucial because cross-feed operations can lead to an imbalance in fuel distribution, which may compromise the aircraft's performance and stability at critical phases of flight. During takeoff and landing, the aircraft requires optimal fuel management to ensure proper weight distribution and to maintain control. Any irregularities can have significant implications for safety.

In contrast, during taxi, cruise, or pre-flight checks, it may be permissible to have cross-feed operations active, as these phases generally do not demand the same level of precision in fuel management as takeoff and landing. However, the regulations and standard operating procedures typically call for cross-feed to be securely turned off during the most critical moments of flight to guarantee optimal control and aircraft integrity.