In which scenario will Field C be vacant according to altitude qualifiers?

The correct choice highlights that Field C will be vacant when neither Mode C nor the controller-entered altitude has been received. This situation indicates a lack of altitude information, which is crucial for accurately monitoring and separating aircraft in controlled airspace. When both data sources are absent, there is no altitude input available for the aircraft's status, resulting in Field C being left open or unfilled.

In the context of aviation operations, understanding the availability of altitude data is essential for safe air traffic control. If altitude information isn't provided, controllers cannot ensure proper separation between aircraft, which is a fundamental aspect of maintaining safety in the airspace.

The other scenarios do not lead to an empty Field C. For instance, in the event of corrupted Mode C reported altitude, while there is an altitude input, it cannot be relied upon due to its compromised integrity. Similarly, if the flight type is IFR, it indicates that the aircraft is following instrument flight rules, which would typically require that they provide valid altitude data. The case of a controller-entered reported altitude that exceeds the assigned altitude indicates a discrepancy but still provides an altitude, even if it's not accurate, leading to Field C containing data rather than being vacant.