What is the main purpose of a transponder in an aircraft cockpit?

The main purpose of a transponder in an aircraft cockpit is to communicate with air traffic control. A transponder serves as a crucial device that transmits the aircraft's location, altitude, and identification information to ground radar systems. This communication enhances situational awareness for both the pilots and air traffic controllers, contributing to safer and more efficient airspace management. The data sent by the transponder helps air traffic control to track and monitor aircraft in real-time, which is particularly important in busy airspaces.

While the other choices may relate to various aspects of aviation, they do not capture the primary function of a transponder. Enhancing passenger comfort, controlling engine performance, and managing onboard systems are important aspects of aircraft operation, but they are not the primary roles of the transponder.