Which instrument is used to measure the aircraft's altitude?

The altimeter is the instrument used to measure an aircraft's altitude. It functions by measuring the air pressure outside the aircraft. As altitude increases, the air pressure decreases; the altimeter translates this drop in pressure into a corresponding altitude reading. This is essential for pilots to maintain proper separation from terrain and other aircraft, and to follow air traffic control instructions.

In contrast, the vertical speed indicator shows the rate of climb or descent in feet per minute but does not provide a specific altitude reading. The airspeed indicator, meanwhile, measures how fast the aircraft is moving through the air but has no relation to altitude. Lastly, the magnetic compass is used for navigation and helps determine the aircraft's heading, but it does not provide any information on altitude. Understanding the primary function of these instruments clarifies why the altimeter is the correct choice for measuring altitude.