Which instrument provides altitude information to air traffic control radar?

The transponder is the correct choice because it is specifically designed to communicate aircraft information, including altitude, to air traffic control (ATC) radar systems. When an aircraft is equipped with a transponder, it sends out a signal that includes data such as the aircraft's identification and its altitude, which is derived from the aircraft's altimeter. This information is crucial for ATC to maintain situational awareness and ensure the safe and efficient movement of air traffic.

On the other hand, while the altimeter provides the pilot with altitude information, it does not transmit that information to radar systems directly. Instead, the altimeter measures the atmospheric pressure to determine the altitude of the aircraft. The vertical speed indicator indicates the rate of climb or descent but does not provide altitude data to ATC. The artificial horizon, or attitude indicator, shows the aircraft's orientation in relation to the horizon, which is essential for instrument flight but does not relate to altitude transmission. Thus, the transponder plays a fundamental role in transmitting altitude information to ATC.