Which instrument indicates a change in altitude?

The altimeter is an essential instrument in aviation that measures altitude by comparing atmospheric pressure outside the aircraft to a calibrated standard. As the aircraft ascends or descends, the pressure decreases or increases respectively. The altimeter converts these changes in pressure into a readable altitude format. Essentially, it provides the pilot with crucial information about the aircraft's height above sea level, which is necessary for maintaining safe flight operations, especially during takeoff, landing, and in controlled airspace.

While the vertical speed indicator also plays a role in altitude awareness by showing the rate of climb or descent, it does not indicate the actual altitude itself. Instead, it reflects changes over time, offering insights into how quickly the aircraft is gaining or losing altitude. Other options such as the airspeed indicator and magnetometer are not related to altitude changes at all; they measure airspeed and magnetic heading, respectively. Thus, the altimeter is the correct choice for indicating a change in altitude.