Shaft/rotor speed is most accurately measured by a

ANS:C - strobometer.

While a stroboscope, also known as a strobometer, can indeed be used to observe and visualize the motion of a rotating object, it is not typically used for accurate measurement of rotational speed. Instead, a stroboscope provides a visual effect that makes a moving object appear to be stationary or moving slowly by flashing light at a frequency synchronized with the object's motion. Here's a more accurate explanation of a stroboscope: A stroboscope, or stroboscopic lamp, is a device that emits short bursts of bright light at regular intervals. These bursts of light are synchronized with the frequency of an object's motion, such as a rotating shaft or machinery component. When the stroboscope flashes light at the same frequency as the object's rotation, the object appears to be stationary or moving slowly, depending on the strobe frequency relative to the object's rotational speed. Stroboscopes are commonly used in industrial settings for visual inspection and analysis of moving machinery, such as rotating shafts, gears, and fans. By adjusting the frequency of the stroboscope's flashes, technicians can freeze the motion of rotating parts, allowing them to inspect the parts for defects, measure speeds, and analyze motion characteristics. However, while stroboscopes provide valuable visual information about the motion of rotating machinery, they are not typically used for precise measurement of rotational speed. For accurate speed measurement, a dedicated instrument such as a tachometer or frequency counter would be more appropriate, as these devices provide quantitative measurements of rotational speed in units such as revolutions per minute (RPM) or hertz (Hz). In summary, while stroboscopes are useful tools for visual observation and analysis of rotating machinery, they are not primarily intended for precise measurement of rotational speed. For accurate speed measurement, dedicated instruments such as tachometers are more suitable.