In the contiguous U.S., at what altitude is an operable coded transponder with Mode C required in all airspace above?

The requirement for an operable coded transponder with Mode C in U.S. airspace is set to enhance safety and improve air traffic control capabilities. In the contiguous U.S., the rule states that above 10,000 feet mean sea level (MSL), a Mode C transponder must be used. This altitude was established to help air traffic control track aircraft more accurately as they operate potentially busy airspace at higher altitudes.

The use of Mode C transponders allows for altitude reporting, which significantly benefits air traffic controllers in maintaining safe vertical spacing between aircraft. While other altitudes mentioned in the choices are significant regarding transponder use in different contexts, such as Class A airspace and certain airspace over densely populated regions, the specific requirement for Mode C above 10,000 feet MSL is outlined in the Federal Aviation Regulations for broader accessibility and safety while flying in the National Airspace System.