In controlled airspace above 10,000 feet MSL, what equipment is required?

In controlled airspace above 10,000 feet MSL, the requirement for a transponder equipped with Mode C capability ensures that an aircraft can provide altitude information to air traffic control (ATC). Mode C transponders not only transmit the aircraft's identification but also provide its altitude information, which is crucial for ATC to maintain safe separation between aircraft. This altitude reporting enhances situational awareness in busy airspace and allows ATC to effectively manage aircraft movements.

The requirement for Mode C specifically is due to the increased density of air traffic and the need for precise altitude data to prevent potential conflicts. This is especially relevant in controlled airspace, where ATC has responsibility for maintaining safe separation between aircraft at differing altitudes.

While other choices may have their own significance, such as the operable coded transponder (which is usually part of what a Mode C transponder encompasses), a radar altimeter and a communication radio are not mandatory for the same purpose in this context. The focus here is on the capability to convey altitude information, which is why Mode C is the correct and specific answer for this scenario.