What signifies that an aircraft is not responding to Mode C?

When an aircraft is not responding to Mode C, the key indicator is that the controller-entered altitude does not equal the single assigned altitude. Mode C transponders are designed to send altitude information to air traffic control, allowing controllers to verify that an aircraft is maintaining or operating at the correct altitude. If there is a discrepancy between what the controller has entered as the assigned altitude and the altitude being reported by the Mode C transponder, it signifies that the aircraft might not be accurately transmitting its altitude information, indicating a potential issue with the Mode C response.

In this context, the effectiveness of altitude reporting is critical for maintaining safe separation between aircraft. Other options do not specifically indicate a failure in Mode C communication. For instance, if the field C displays operational altitude or if an aircraft has reported effective altitude, that could simply mean that the aircraft is responding appropriately. Similarly, a match between controller-entered altitude and assigned altitude does not provide any clear indication of Mode C issues either, as it suggests that the aircraft's altitude aligns with what is expected. Therefore, when the controller's altitude input does not match the expected value, it clearly points to a failure or lack of response from the aircraft's Mode C transponder.