Strain is defined as:

Strain is fundamentally understood as the ratio of the change in length (extension) of a material to its original length. This definition captures how much a material deforms compared to its initial state when subjected to stress. Strain is a dimensionless quantity, often expressed as a percentage or a fraction, and provides insight into the material's deformation characteristics under various loading conditions.

Understanding strain in this manner is crucial because it allows engineers and materials scientists to assess how materials will behave when forces are applied. This knowledge is vital for ensuring safety and performance in engineering applications.

The other options, while relevant to material properties, do not accurately define strain. For instance, the ratio of load to stress pertains to different mechanical concepts, and while a material's elasticity is associated with how it returns to its original shape after deformation, it does not define strain. Similarly, maximum deformation refers to the extent of change in dimensions but does not offer the relative measure that strain does. Thus, the option that states strain is the ratio of extension to original length correctly captures the essence of this fundamental mechanical property.