ANS:A - shaft encoder.

Angular displacement can be measured suitably by a shaft encoder. A shaft encoder is a device that converts the angular position or rotation of a shaft into an electronic signal. It typically consists of a rotating disk with slots or marks (optical encoder) or a series of magnetic sensors (magnetic encoder), along with associated electronics to detect the position changes. When the shaft rotates, the encoder detects the movement of the slots or marks (in the case of an optical encoder) or the changes in the magnetic field (in the case of a magnetic encoder), and produces electrical signals corresponding to the angular displacement. These signals can then be processed by a controller or computer to determine the angular position, speed, or direction of rotation of the shaft. Therefore, a shaft encoder is a suitable device for measuring angular displacement accurately and precisely.