What type of data does a Magnetometer measure?

A magnetometer measures the Earth's magnetic field, which is crucial for navigation purposes. It detects the strength and direction of magnetic fields, allowing the aircraft's navigation system to determine its orientation relative to the Earth's magnetic field. This information is essential for calculating headings and ensuring accurate navigation, particularly when used in conjunction with other systems like inertial navigation or GPS.

The other options involve measurements that are not related to magnetic fields. For instance, measuring aircraft speed pertains to airspeed indicators, fuel levels are monitored through fuel gauges, and air pressure in the cockpit is typically measured by altimeters and pressure sensors, none of which utilize the magnetometer's capabilities.