Answer: D. Joints
The mechanism that allows relative movement between the parts of the robot arm is called a joint.
In industrial robots, joints are important components that connect various components, which allow relative movement between parts of the robot arm, thus forming a complete motion chain and realizing various complex motion forms.
Joints are generally composed of transmission mechanisms, controllers, sensors, etc., which work together to enable flexible relative movements between the various components of the robot arm to meet various operational needs. Therefore, the answer to this question is D.