The correct answer is B) Critical speed. Critical speed refers to the operational speed at which excessive engine vibration is created. This is a characteristic of the engine design and is the speed at which the engine's natural frequency matches the engine's rotational frequency, leading to resonant vibrations. Operating the engine at this critical speed should be avoided as it can cause significant mechanical stress and damage to the engine components. The other options are incorrect because: A) Non-harmonic speed is not a standard term related to engine vibration. C) Maximum speed is the highest allowable speed of the engine, which may or may not coincide with the critical speed. D) Design maximum speed is the maximum speed specified by the engine manufacturer, which again may not be the same as the critical speed.