Which of the following rotational shaft speed sensors produces a digital signal?

Correct Answer: Option A — Optical tachometer

Optical tachometers utilize a light source and a photodetector to count reflections from a rotating target. This process generates discrete pulses, creating a digital signal. Conversely, DC and AC tachometers are analog devices that produce a voltage proportional to rotational speed, requiring conversion for digital processing in modern control systems.