Proof stress is related to Elongation.

Proof stress is a measure of the maximum stress that a material can withstand without exhibiting permanent deformation. It is also known as the yield strength or yield stress of the material.

Yielding:
- Yielding refers to the point at which a material starts to deform permanently under stress.
- The proof stress is the stress at which yielding occurs, indicating the point at which the material begins to show plastic deformation.
- It is an important characteristic of materials as it helps determine their mechanical properties and their ability to withstand loads.

Elongation:
- Elongation is a measure of the extent to which a material can stretch or deform under tensile stress before breaking.
- When a material is subjected to stress beyond its proof stress, it undergoes elongation as it is stretched beyond its original dimensions.
- The amount of elongation a material can undergo before breaking is an important property to consider in engineering applications.

Relationship between Proof Stress and Elongation:
- As the proof stress of a material increases, its ability to withstand stress without permanent deformation also increases.
- This means that materials with higher proof stresses can undergo greater elongation before breaking.
- On the other hand, materials with lower proof stresses have lower elongation capabilities and are more likely to deform or break under stress.

Conclusion:
- The correct answer, option 'D', shows that proof stress is related to elongation.
- The proof stress determines the point at which a material starts to deform permanently, while elongation measures the extent to which a material can stretch before breaking.
- Therefore, the higher the proof stress, the greater the elongation capability of a material.