What role does the tachometer serve in an aircraft?

The tachometer plays a crucial role in monitoring the engine's performance by indicating the revolutions per minute (RPM) of the engine. This measurement is vital for pilots because it helps them understand how fast the engine is operating, which is essential for managing power output and ensuring the engine stays within safe operational limits. Properly maintaining engine RPM is important for performance, efficiency, and the overall safety of the flight.

While the other options involve important measurements in aviation, they serve different functions. Airspeed, fuel levels, and altitude readings are indicated by other instruments, such as the airspeed indicator, fuel gauge, and altimeter. The tachometer, however, is specifically designed to provide information regarding the engine's rotational speed, making answer B the correct choice for this question.