What aspect defines the operation of the PSUs during flights?

The correct answer is that PSUs (Passenger Service Units) are always available for passenger use during flights. This is significant because PSUs play a critical role in providing passengers with essential services such as reading lights, air vents, and the call button for assistance. By being available throughout the flight, they enhance the overall passenger experience and allow for comfort and accessibility at any time.

In contrast, options suggesting limited operation during takeoff and landing, solely under crew control, or functionality only during emergencies are misleading. While crew control might indicate their management for safety protocols, it does not reflect the continuous availability for passengers throughout the flight duration. Recognizing that PSUs are integrated into the inflight experience is vital for understanding their importance in passenger service and safety.