In mechanical engineering, the concept of material strength is of great importance. One way to measure the strength of a material is through a tension test, which is commonly used to determine the mechanical properties of materials. In a tension test, a material sample is subjected to an increasing tensile force until it breaks. During this test, two kinds of notches can be observed: the elastic limit and the ultimate stress.

Elastic Limit:
The elastic limit is the maximum stress that a material can withstand without experiencing permanent deformation. At this point, the material is still able to return to its original shape once the load is removed. It represents the boundary between the elastic region and the plastic region of the stress-strain curve.

Ultimate Stress:
The ultimate stress, also known as the ultimate strength or tensile strength, is the maximum stress that a material can withstand before it fractures or breaks. This is the point at which the material experiences permanent deformation and cannot return to its original shape. The ultimate stress is typically higher than the elastic limit and is an important parameter in determining the suitability of a material for specific applications.

Fracture Stress:
The fracture stress refers to the stress at which a material breaks or fractures. It is the stress value at the point of failure in the stress-strain curve. Fracture stress is dependent on various factors such as the material's microstructure, impurities, and the presence of defects or notches.

Difference between Elastic Limit and Ultimate Stress:
The main difference between the elastic limit and ultimate stress is that the elastic limit represents the maximum stress a material can withstand without permanent deformation, while the ultimate stress represents the maximum stress a material can withstand before it fractures or breaks.

In summary, the correct answer to the given question is option 'C' - ultimate stress, as it is in a tension test where two kinds of notches, namely the elastic limit and the ultimate stress, can be observed.