Which of the following should be confirmed for an aircraft at or above FL180 before validating the Mode C readout?

For an aircraft operating at or above FL180, it is essential to confirm the pilot's altitude reporting before validating the Mode C readout. At this altitude, accurate altitude reporting is critical for maintaining safe separation between aircraft, as altitude discrepancies can lead to dangerous situations such as mid-air collisions.

Mode C transponders provide altitude information to air traffic control, and this information must be verified to ensure that the aircraft is flying at the correct altitude as stated by the pilot. Ensuring the accuracy of the altitude reporting is vital, as it directly affects flight safety and the effectiveness of air traffic management.

The other options, while relevant to overall flight operations, do not have the same immediate importance in the context of validating the Mode C readout specifically. While the navigation system, weather conditions, and flight plan details are important for flight management and planning, they do not directly pertain to the critical task of ensuring that a pilot is reporting their altitude correctly while operating in controlled airspace at higher altitudes.