C

Step 1: Understand the types of strains in material science. Strain is defined as the deformation per unit length caused by applied stress.
Step 2: Identify the types of strains.
- Longitudinal strain: This occurs when an object is subjected to tensile or compressive forces along its length, leading to a change in length. It is expressed as: $\text{Longitudinal Strain} = \frac{\Delta L}{L_0}$, where $\Delta L$ is the change in length and $L_0$ is the original length.
- Shearing strain: This is the change in shape of an object due to forces acting parallel to a surface rather than along its length.
- Volume stress: This refers to changes in volume due to applied uniform pressure but does not necessarily change the object's overall shape.
- Metallic strain: This is not a standard term used in the context of strain types but may refer to the behavior of metallic materials under stress.
Step 3: Since the question specifically asks for the reason behind the change in the shape of a regular body, the most relevant option is: Longitudinal strain, as it encompasses deformation along a length that affects the shape.
Therefore, the correct answer is option C.