Strain energy is a concept used in the field of mechanical engineering to analyze and understand the behavior of materials under applied loads. It is a form of potential energy that is stored within a material when it is deformed or strained.

The correct answer to the given question is option 'C', which states that strain energy is used to find deflection. This is because strain energy is directly related to the amount of deformation or deflection that a material undergoes when subjected to an external force.

Here is a detailed explanation of why strain energy is used to find deflection:

1. Strain Energy:
Strain energy is the energy stored within a material when it is subjected to external forces. It is a result of the elastic deformation of the material. When a material is loaded, it deforms and stores energy in the form of potential energy within its structure. This energy is referred to as strain energy.

2. Relationship between Strain Energy and Deflection:
The amount of strain energy stored in a material is directly related to the amount of deformation or deflection that the material undergoes. As the material deforms, its molecules are displaced from their original positions, causing a change in the internal potential energy. This change in potential energy is quantified as strain energy.

3. Deflection Calculation:
Deflection refers to the displacement or bending that a material undergoes when subjected to an external load. It is an important parameter to consider in structural analysis and design. By calculating the strain energy stored in a material, the deflection can be determined.

4. Strain Energy Method:
The strain energy method is a commonly used approach to calculate deflection in engineering structures. It involves determining the potential energy stored within a material due to deformation and equating it to the work done by the external loads. By solving this equation, the deflection of the structure can be determined.

Conclusion:
In conclusion, strain energy is used to find deflection in mechanical engineering. It is a measure of the potential energy stored within a material due to deformation. By calculating the strain energy, the amount of deflection that a material undergoes can be determined. This information is crucial in the analysis and design of engineering structures.