In what altitude structure is a transponder required when operating in controlled airspace?

In controlled airspace, the requirement for transponder usage relates specifically to altitude and airspace classifications. A transponder is required above 10,000 feet mean sea level (MSL), but this requirement does not apply to the airspace below 2,500 feet above ground level (AGL). This is crucial for maintaining situational awareness among air traffic control and other aircraft operating in the vicinity.

When an aircraft operates at altitudes above 10,000 feet MSL, the transponder provides vital information about the aircraft's altitude, its location, and its identification to air traffic control, thus enhancing safety. Notably, pilots flying within controlled airspace below 2,500 feet AGL may not face transponder requirements unless in a specific type of airspace where such a requirement is enforced. Therefore, the correct answer reflects this altitude structure and the operational requirements set by aviation regulations.