What is the function of the altimeter in an aircraft?

The altimeter in an aircraft serves the essential function of measuring altitude based on atmospheric pressure. It operates on the principle that air pressure decreases with an increase in altitude. As the aircraft ascends, the atmospheric pressure drops, and the altimeter converts this pressure change into an altitude reading. This information is crucial for pilots to maintain safe flying altitudes, comply with air traffic control instructions, and ensure safe terrain clearance.

Other functions suggested in the options involve different instruments. For instance, measuring vertical speed during climb and descent is typically the role of a vertical speed indicator, while fuel levels in the tank are monitored by fuel gauges. Navigation through GPS coordinates is managed by the aircraft's navigation system rather than the altimeter. Therefore, the correct answer highlights the specific and vital role of the altimeter in gauging altitude, which is critical for the safe operation of the aircraft.