What electronic device measures the magnetic field around the aircraft?

The correct choice is the magnetometer, as it is specifically designed to measure the strength and direction of the magnetic field in the environment surrounding the aircraft. This device helps in determining the aircraft's orientation relative to the Earth's magnetic field, which is crucial for navigation purposes.

Magnetometers work by detecting changes in the magnetic field that can inform a flight control system about the aircraft's position and heading. This is essential for maintaining stable flight and correct navigation, especially when GPS signals may be compromised or unavailable.

In contrast, an altimeter measures altitude by detecting changes in air pressure, a gyroscope provides information about the aircraft's rotational movements, and a barometer measures atmospheric pressure, which can assist in altitude determination but does not provide information about the magnetic environment. Each of these devices has its distinct function in aviation, but the magnetometer is unique in its specific role of measuring the magnetic field.