If ATC requests a speed adjustment beyond the aircraft's limits, what must the pilot do?

Answer: C — Advise ATC of the airspeed that will be used.

When air traffic control requests a speed adjustment that exceeds the aircraft's operational limits, the most appropriate course of action is for the pilot to inform ATC of the maximum airspeed the aircraft can safely maintain. This communication is crucial because it ensures that air traffic control is aware of any limitations the aircraft has that may impact its ability to comply with requests. By advising ATC of the airspeed that will be used, the pilot can still work collaboratively with air traffic control while maintaining the safety and integrity of the flight. In this situation, it is essential for pilots to recognize that safety overrides compliance with ATC requests when such requests are beyond the aircraft's performance capabilities. Proper communication helps establish a shared understanding, allowing ATC to make any necessary adjustments to their instructions, ensuring overall safety in the airspace.