Which condition requires that Mode C altitude is reported and still lacks an assigned altitude?

The requirement for Mode C altitude reporting, combined with the absence of an assigned altitude, aligns with the situation in which Field C indicates "X." This designation typically signifies that the aircraft is operating in a scenario where the Mode C data is essential for air traffic management, particularly in controlled airspace where altitude awareness is crucial for separation and traffic control.

In scenarios involving the use of transponders, the "X" in Field C indicates that the aircraft is maintaining a valid Mode C altitude reporting but has not been provided with a specific altitude instruction from air traffic control. Understanding this distinction is vital for air traffic controllers as it denotes that, while the aircraft is equipped to report its altitude, it is currently subjected to a situation where its altitude has not been explicitly directed, allowing controllers to monitor the aircraft's vertical position relative to other traffic effectively.

Other options do not accurately capture the specific circumstance of a Mode C requirement lacking an assigned altitude. For instance, populating Field C or leaving it empty would not appropriately indicate the operating status regarding the required reporting of altitude when the specific direction has not been given. It's essential for those in air traffic control to recognize the implications of these designations to ensure safety and efficiency in managing flight operations.