What is the function of Mode C in altitude reporting?

The function of Mode C in altitude reporting is to provide the aircraft's altitude as transmitted by the Mode C transponder. Mode C transponders are equipped to send altitude information along with the aircraft's identification. When an aircraft is equipped with a Mode C transponder, it automatically transmits its pressure altitude information to air traffic control and nearby aircraft, which enhances situational awareness and aids in maintaining safe separation.

While other options may relate to various capabilities of transponders or other navigation and communication systems, they do not accurately represent the primary purpose of Mode C operations. Mode C specifically adds value to altitude reporting and compliance with air traffic control requirements by providing precise altitude information that is vital for managing traffic in controlled airspace.