Which of the following best defines the concept of 'straining' in materials science?

The concept of 'straining' in materials science specifically refers to the change in length per unit length of a material when subjected to stress. This definition captures how materials deform under load and provides a measure of the extent of that deformation. Strain is a dimensionless quantity, which signifies the relative change in size or shape when stress is applied, making option C the most accurate and relevant choice in the context of straining.

In materials science, understanding strain is critical because it directly relates to the material's elasticity and plasticity, which are important for assessing how materials will behave under various loads and conditions. By knowing the strain, engineers and architects can predict how structures will respond and ensure stability and safety in their designs.