The correct answer is option 'D', which states that the property of a material that cannot be significantly changed by heat treatment is the elastic modulus. Let's delve into the explanation below:

Heat treatment is a process that involves the controlled heating and cooling of a material to alter its properties. It is commonly used to improve mechanical properties such as strength, hardness, and ductility. However, there are certain properties of a material that remain relatively unaffected by heat treatment, and one of those properties is the elastic modulus.

The elastic modulus, also known as Young's modulus, is a measure of a material's stiffness or rigidity. It describes how much a material deforms under a given amount of stress. Mathematically, it is defined as the ratio of stress to strain within the elastic range of a material.

The elastic modulus is primarily determined by the nature of the atomic and molecular bonds within a material. These bonds define the material's inherent stiffness and are not significantly affected by heat treatment. Therefore, regardless of the heat treatment process applied, the elastic modulus of a material remains relatively constant.

Other material properties, such as yield strength, ultimate tensile strength, and ductility, can be significantly changed by heat treatment.

- Yield strength is the amount of stress a material can withstand before it starts to deform plastically. Heat treatment can alter the yield strength by modifying the material's microstructure, such as grain size and dislocation density.
- Ultimate tensile strength is the maximum stress a material can withstand before it fractures. Heat treatment can enhance the ultimate tensile strength by promoting the formation of stronger phases or by refining the microstructure.
- Ductility is the ability of a material to undergo plastic deformation before fracturing. Heat treatment can increase or decrease the ductility of a material depending on the specific heat treatment process employed.

In summary, while properties such as yield strength, ultimate tensile strength, and ductility can be significantly changed by heat treatment, the elastic modulus remains relatively constant regardless of the heat treatment process.