What must be monitored if neither controller-entered nor Mode C reported altitude has been received?

In the context of air traffic control, if neither controller-entered nor Mode C reported altitude has been received, it is crucial to understand the implications of receiving no altitude information. When altitude data is missing, Field C, which is used for reporting altitude information, will be vacant.

This vacancy signifies that there is no available altitude data for the flight, which could impact separation and safety protocols. Controllers rely on accurate altitude data to manage aircraft safely, so the absence of this data necessitates further actions, such as requesting altitude reports from the pilot directly or implementing alternative procedures.

Understanding that Field C will be vacant highlights the importance of effective communication and monitoring procedures in air traffic control operations, particularly when dealing with altitude verification. The other options pertain to aspects that would either not apply in this scenario or assume that some altitudinal data would be available, which isn't the case here.