Which instrument is used to measure altitude and often functions as an aneroid barometer?

The correct instrument for measuring altitude that also functions as an aneroid barometer is the altimeter. An altimeter operates by measuring the atmospheric pressure; as altitude increases, the pressure decreases. This relationship allows the altimeter to provide readings of an aircraft's altitude above sea level based on the current air pressure.

In aviation, accurate altitude measurement is crucial for safe flying, as it helps pilots maintain proper clearance above the ground and other aircraft. The altimeter is specifically calibrated to convert changes in pressure into altitude readings, allowing for effective navigation and adherence to airspace regulations.

In contrast, a gyroscope is primarily used for maintaining orientation and can assist with navigation and stability, but it does not measure altitude. The heading indicator provides information on an aircraft's direction in relation to magnetic north but also does not measure altitude. The magnetic compass indicates direction based on the Earth's magnetic field and is unrelated to altitude measurement.