What instrument monitors propeller speed (Np)?

The correct answer, which indicates that a phase shift torque probe monitors propeller speed (Np), highlights the instrument's ability to measure the phase difference between the torque applied and the torque measured during operation. This phase relationship is essential in determining the rotational speed of the propeller in propeller-driven aircraft or systems, as it allows for accurate monitoring and control of propeller dynamics.

It's important to consider that the other options, while potentially related to engine performance or torque measurement, do not specifically focus on the direct monitoring of propeller speed. A torque gauge, for example, measures the amount of torque generated but does not directly relate to speed measurement. An RPM counter measures rotational speed but typically focuses on engine or rotor speeds rather than specifically correlating to propeller dynamics. Similarly, an engine performance monitor provides broader performance metrics without concentrating solely on propeller speed. Thus, the phase shift torque probe is uniquely suited to this specific monitoring purpose.