The knuckle joint is a type of mechanical joint used to connect two rods or bars in such a way that they can rotate relative to each other. It consists of several important parts that work together to allow this rotational movement. The important parts of a knuckle joint include:

1. Eye: The eye is the circular or elliptical hole in one of the bars that receives the other bar. It provides a means of connection and allows for the rotation of the joint.

2. Pin: The pin is a cylindrical component that passes through the eyes of the two bars and holds them together. It acts as a pivot point for the rotational movement of the joint.

3. Fork: The fork is the part of the joint that is formed by the two arms or branches of one of the bars. It surrounds the other bar and provides support and guidance for its movement.

These three parts, namely the eye, pin, and fork, are the essential components of a knuckle joint. They work together to allow the desired rotational movement between the connected bars. The eye provides the connection, the pin acts as the pivot, and the fork guides the movement.

The correct answer is option 'D' - each of the mentioned. All the mentioned parts are important and necessary for the proper functioning of a knuckle joint. Without any one of these parts, the joint would not be able to perform its intended function effectively.

In conclusion, the eye, pin, and fork are all important parts of a knuckle joint. They work together to enable the rotational movement between the connected bars, making the joint a versatile and widely used mechanical component.