Permanent set refers to
(A) Elastic strain in the material below the elastic limit
(B) Elastic strain in material beyond the elastic limit
(C) Plastic strain in the material below the elastic limit
(D) Plastic strain in material beyond the elastic limit

Hint When an external force acts on a body and deforms it, there will be an internal restoring force that is developed within the body and it opposes the deforming force. This will produce stress in the material. The restoring force developed in the material per unit area is called stress. Stress will lead to a change in the length or shape or volume of the body.

Complete step by step solution:
The ratio of change in the dimension of a body to the original dimension of the body is called strain.
If the deformed body returns to its original state after the removal of the deforming force, then such strain is called elastic strain.
Plastic strain is a kind of permanent strain, where even after the removal of the deforming force the body will not return to its original state.
Permanent set is an irreversible deformation in a material due to its exposure to stress.
Hence permanent sets can be referred to as the plastic strain in a material beyond the elastic limit.

The answer is: Option (D): Plastic strain in a material beyond the elastic limit.

Note:
There are three types of strains they are linear strain, volume strain, and shearing strain. The ratio of change in length of a material to its original length is called linear strain. The ratio of change in volume to the original volume is called volume strain and the ratio of change in shape to the original shape is called shearing strain. Below the elastic limit, the stress and strain are proportional hence the material will regain its original state as soon as the deforming force is removed. Even beyond the elastic limit for a certain range, the material will regain its original state. The point till the material can be exposed to force and still it will regain its original shape is called the elastic limit or the yield point. Beyond this point, the material will not regain its original shape. It will be permanently deformed.