Which instrument would you use to measure altitude?

The altimeter is the instrument specifically designed to measure altitude. It works by detecting changes in atmospheric pressure; as an aircraft ascends or descends, the outside air pressure changes, and the altimeter translates these changes into altitude readings. This instrument is crucial for maintaining appropriate flight levels and ensuring safe separation between aircraft, especially in controlled airspace.

The other instruments listed serve different functions in aviation. The airspeed indicator measures the velocity of the aircraft relative to the surrounding air, which is important for controlling speed but does not provide altitude information. The vertical speed indicator shows the rate of climb or descent, indicating how fast the aircraft is ascending or descending but not the actual altitude. The heading indicator provides information about the direction in which the aircraft is pointed but does not relate to altitude at all.

Thus, the use of an altimeter is essential for determining altitude, making it the correct choice.