Plasticity refers to the ability of a material to undergo permanent deformation without fracturing or breaking. It is an important property of materials, especially in the field of mechanical engineering. Plasticity is typically observed in ductile materials, such as metals, which can be easily deformed under certain conditions.

Let's break down the given options and explain why all of them are true about plasticity:

a) Permanent Deformation:
Plastic deformation refers to the change in shape or size of a material that is not reversible once the applied load is removed. This means that the material retains its deformed shape even after the load is no longer present. This is in contrast to elastic deformation, where the material returns to its original shape after the load is removed. Therefore, plasticity involves permanent deformation.

b) Ability to retain deformation under load or after removal of load:
Plasticity allows a material to retain its deformed shape even when the load is applied or after the load is removed. This means that the material can maintain its new shape without springing back to its original form. This ability is crucial in applications where the material needs to withstand loads or maintain a specific shape under different conditions.

c) Plastic deformation is greater than elastic deformation:
Plastic deformation is typically greater than elastic deformation in ductile materials. When a load is applied to a material, it first undergoes elastic deformation, where it can return to its original shape once the load is removed. However, if the load exceeds a certain threshold, plastic deformation occurs, leading to permanent changes in the material's shape. The amount of plastic deformation is generally greater than the elastic deformation experienced by the material.

d) All of the mentioned:
Based on the explanations above, all of the mentioned options are true about plasticity. Plasticity involves permanent deformation, the ability to retain deformation under load or after removal of the load, and plastic deformation being greater than elastic deformation.

In conclusion, plasticity is an essential property of materials, particularly in mechanical engineering. It allows materials to undergo permanent deformation, retain their deformed shape under load or after the load is removed, and exhibits greater plastic deformation compared to elastic deformation.