Which instrument is used to display an aircraft's altitude?

The instrument used to display an aircraft's altitude is the altimeter. The altimeter measures the atmospheric pressure and translates this pressure into an altitude reading. This is crucial for pilots, as it helps them maintain a safe and controlled altitude during flight. The altimeter is typically calibrated to a standard atmospheric pressure at sea level, allowing it to provide accurate altitude readings as the aircraft ascends or descends.

The other instruments listed serve different functions. A speedometer measures the aircraft's speed, which is essential for understanding performance during takeoff, flight, and landing. A gyroscope is used for maintaining orientation and stability, playing a vital role in navigation and control but does not provide altitude information. A barometer measures air pressure but is not designed specifically for aviation use; while it is related to altitude measurement, it does not give a direct altitude readout like an altimeter does.