What is the system that utilizes a ground-based interrogator and transponder?

The correct choice is secondary radar, which operates by having a ground-based interrogator that sends out signals to aircraft transponders. When the transponder receives the signal from the interrogator, it responds with information that includes the aircraft's identity and altitude, allowing for more accurate tracking than primary radar can provide. Secondary radar is important in air traffic control as it enhances situational awareness and facilitates the management of air traffic by providing real-time data about each aircraft's status.

Primary radar, in contrast, relies solely on reflected radio waves from the aircraft without any input from the aircraft itself, resulting in less detailed information. Visual Flight Rules (VFR) pertain to regulations for flying under visual meteorological conditions and do not involve radar systems directly. Ground Control Approach (GCA) is a procedure used by air traffic controllers to guide aircraft during landing, which can use various radar systems, but it is not defined primarily as a system utilizing interrogators and transponders like secondary radar is.