What tool is commonly used to measure engine RPM in aircraft?

The tachometer is the correct tool for measuring engine RPM (revolutions per minute) in aircraft. This instrument provides critical information about the engine's operational performance by indicating how fast the engine is rotating. Tachometers are essential for pilots and maintenance personnel to monitor engine performance, ensuring that it operates within specified RPM ranges, which is crucial for engine efficiency and safety.

The other options listed do not serve the purpose of measuring engine RPM. For example, a speedometer measures the speed of the aircraft in flight, while a manifold gauge is used to measure pressure levels in a system, such as the manifold pressure within an engine. An altimeter measures altitude by determining the atmospheric pressure surrounding the aircraft, which is entirely unrelated to engine RPM. Therefore, the tachometer is uniquely designed to fulfill the need for RPM readings, making it an essential component in aircraft instrumentation.