The correct answer is A) Optical tachometer. An optical tachometer produces a digital signal, which is a key advantage over other types of rotational shaft speed sensors. The digital signal is generated by an optical sensor that detects the passing of reflective markings on the rotating shaft, producing a series of digital pulses that correspond to the shaft speed. This digital output is well-suited for interfacing with modern electronic control systems. In contrast, the other options (B, C, and D) utilize analog signals that change proportionally to the shaft speed, which may require additional signal conditioning or analog-to-digital conversion for use with digital control systems. The optical tachometer's digital output makes it the preferred choice for rotational shaft speed sensing in many applications.