Which instrument displays the aircraft's altitude?

The altimeter is the instrument responsible for displaying the aircraft's altitude. It measures the atmospheric pressure and converts this information into an altitude reading. As the aircraft ascends or descends, changes in air pressure are detected by the altimeter, which provides the pilot with a clear indication of how high the aircraft is flying above sea level or the ground level, depending on the setting and type of altimeter used.

In contrast, the vertical speed indicator measures the rate of climb or descent, providing information about how quickly the airplane is changing altitude, rather than the actual altitude itself. The airspeed indicator displays the speed of the aircraft relative to the surrounding air, while the heading indicator shows the direction the aircraft is facing. Each of these instruments plays a vital role in flight operations, but only the altimeter is specifically designed to report altitude, making it the correct choice.