What system serves as a self-contained airborne collision avoidance system intended to back up ATC separation services?

The correct choice, TCAS (Traffic Alert and Collision Avoidance System), is designed specifically to provide pilots with warnings about potential mid-air collisions. It functions as an autonomous system aboard the aircraft that continuously monitors the airspace surrounding it for other transponder-equipped aircraft. If potential collision risks are detected, TCAS prompts the flight crew with visual alerts and, if necessary, provides resolution advisories that instruct the pilots on how to avoid a collision, such as climbing or descending to a safe altitude.

This system operates independently of air traffic control services, hence serving as a critical backup for maintaining safe separation between aircraft. It is particularly vital in situations where ATC might not have full situational awareness of every aircraft in a busy airspace.

Other options presented, such as ADS-B (Automatic Dependent Surveillance–Broadcast), ILS (Instrument Landing System), and RAIM (Receiver Autonomous Integrity Monitoring), serve different purposes and are not primarily collision avoidance systems. ADS-B provides positional information and enhances situational awareness for both pilots and air traffic controllers, but it does not actively prevent collisions. ILS is used for precise landings and navigation, while RAIM ensures the integrity of GPS signals but does not involve collision avoidance. Therefore, TCAS stands out as