When is a Mode C transponder required?

A Mode C transponder is required in a variety of airspace scenarios to ensure proper surveillance of aircraft, aiding in preventing collisions and enhancing situational awareness among pilots and air traffic control.

It is specifically required in and above Class B airspace to provide additional information on aircraft position and altitude. This requirement helps maintain safe separation in the controlled airspace near major airports, where traffic density is typically high.

Additionally, above 10,000 feet MSL (Mean Sea Level), a Mode C transponder is needed unless the aircraft is below 2,500 feet AGL (Above Ground Level). This regulation is in place to assist air traffic control in tracking aircraft and managing the increased potential for traffic conflicts at higher altitudes, where airspace usage often includes commercial and general aviation traffic.

Considering that these requirements apply cumulatively to various airspace classifications, the choice that encompasses all scenarios where a Mode C transponder is mandated accurately reflects the regulations, making it the correct answer.