The correct answer for the mechanism used in a shaping machine is option 'D', which states that it is an inversion of the single slider-crank chain. In order to understand this answer, let's break it down and explain each part in detail.

**Mechanism in a Shaping Machine**
A shaping machine is a type of machine tool used in manufacturing processes to produce flat surfaces, keyways, slots, and other shapes by removing material from a workpiece. It consists of a cutting tool that reciprocates in a linear motion, and the workpiece is held in a vice or fixture that moves in a controlled manner.

**Inversion of the Single Slider-Crank Chain**
The single slider-crank chain is a four-bar mechanism that consists of three revolute joints and one sliding joint. It is commonly found in engines and machines that convert rotary motion into reciprocating motion. In this mechanism, a rotating crank drives a connecting rod, which in turn drives a slider in a linear motion.

The inversion of a mechanism is a different configuration of the same basic elements. It involves fixing different links of the four-bar chain to create a new mechanism. In the case of a shaping machine, the single slider-crank chain is inverted to create a different configuration that is suitable for the desired application.

**Why is it an Inversion?**
The shaping machine mechanism is an inversion of the single slider-crank chain because the slider, which is the cutting tool, is fixed, and the crank is driven by a rotating motor. In a traditional single slider-crank chain, the crank is driven by a motor, and the slider moves back and forth. In the inversion used in shaping machines, the slider is fixed, and the crank moves in a circular motion.

This inversion allows for precise control of the cutting tool's motion, as the fixed slider provides stability and accuracy. The workpiece is held in a vice or fixture, which is attached to the moving link of the mechanism. As the crank rotates, it moves the fixture and the workpiece in a controlled linear motion, allowing the cutting tool to remove material from the workpiece in a desired shape.

Overall, the shaping machine mechanism is an inversion of the single slider-crank chain because it modifies the configuration of the basic elements to suit the specific requirements of shaping operations. This inversion provides the necessary control and accuracy for shaping processes, making it an ideal choice for the machine.