What function does the altimeter serve in an aircraft?

The altimeter's primary function in an aircraft is to measure altitude. It does this by using the principle of atmospheric pressure to determine how high the aircraft is flying above sea level. The altimeter measures the ambient air pressure and compares it to a standard pressure, allowing the pilot to know the altitude relative to sea level. This is crucial for navigation, ensuring safe terrain clearance, and maintaining proper vertical separation from other aircraft during flight.

Options indicating speed, fuel usage, or engine performance represent other important parameters in aircraft operation, but they do not pertain to altitude measurement. Speed is indicated by an airspeed indicator, fuel usage is monitored through fuel flow gauges, and engine performance is assessed through engine instruments such as RPM gauges and temperature indicators. Thus, these functions are distinct from the role of the altimeter, which is exclusively focused on providing altitude information.