What is a transponder in aviation?

A transponder in aviation is an aircraft receiver-transmitter that sends coded signals, which is crucial for air traffic control and surveillance systems. When an aircraft's transponder is activated, it responds to radar interrogations with a unique identification code, along with information about its altitude. This coded signal helps air traffic controllers identify and track aircraft on their radar screens, contributing to overall flight safety by reducing the risk of mid-air collisions.

The other choices represent different aviation tools or systems that serve distinct functions. For instance, a navigation tool for measuring altitude typically refers to altimeters or other instruments that provide altitude data but do not transmit identification signals. Instruments for flight information display are used to present flight data to pilots but do not play a role in identifying the aircraft. Lastly, a system that manages aircraft weight pertains to load management and is critical for flight performance but does not involve the transmission of coded signals essential for tracking by air traffic control. Thus, the correct characterization of a transponder aligns with its primary function as a receiver-transmitter of coded signals, enabling effective communication and identification in the aviation environment.