In a coupling rod of a locomotive, each of the four pairs is a ________ pair.

The correct answer is (B) Turning.
Let's break down why:

What is a Kinematic Pair (or just Pair)?
A kinematic pair is a connection between two links (or elements) that allows them to move relative to each other in a specific way.

Understanding the Options:
* Sliding Pair: This pair allows one link to slide along another (like a piston in a cylinder).
* Turning Pair: This pair allows one link to turn or rotate about a fixed axis on another link (like a hinge).
* Rolling Pair: This pair allows one link to roll over another (like a wheel on a road).
* Screw Pair: This pair combines turning and sliding motion (like a screw in a nut).

Why Turning Pair is the Answer for a Coupling Rod:
A coupling rod connects the wheels of a locomotive.
At each end of the coupling rod, there's a connection point (usually a pin) that allows the rod to rotate relative to the wheel or another connecting rod.
The wheels rotate, and the coupling rod rotates along with the wheel in a circular motion around the wheel's center.
This rotational motion is best described by a turning pair.
Therefore, each of the four pairs in a coupling rod of a locomotive is a turning pair.