The reason for the change in shape of a regular body is

The correct option is Shearing strain.

To understand why this is the case, let us examine how different types of strain affect a physical body:
1. Longitudinal Strain: This is produced when a deforming force acts along the length of a wire or bar, causing a change in length per unit original length.
2. Volume Strain: This is produced when a uniform force acts normally over the entire surface of a body, changing its volume but maintaining its original shape.
3. Shearing Strain: This occurs when a tangential deforming force is applied parallel to one face of a body while the opposite face is held fixed. This causes a relative displacement between the layers of the body, leading to a change in its geometric shape (for example, a rectangle turning into a parallelogram) without any change in its volume. The magnitude of this deformation is represented by the angle of shear ($\theta$).

Therefore, the change in the shape of a regular body is due to shearing strain.