Under what conditions is a transponder required to be operable?

A transponder is required to be operable when flying in controlled airspace or above 10,000 feet. This requirement is in place to enhance the safety of the National Airspace System. In controlled airspace, the use of a transponder allows air traffic control to identify and communicate with aircraft, which is essential for maintaining safe separation and facilitating traffic management.

Furthermore, at altitudes above 10,000 feet, even outside of controlled airspace, the increased traffic volume and the potential for encounters with other aircraft make it critical for all pilots to have transponder functionality. This ensures that their aircraft can be tracked and monitored for safety purposes.

If a pilot operates the aircraft in uncontrolled airspace or at lower altitudes, they are not required to have a transponder necessarily, but using one is considered good practice and enhances situational awareness. The other options do not accurately represent the regulatory requirements regarding transponder usage in the National Airspace System.