What defines a material's ability to deform elastically and return to its original shape when the load is removed?

The ability of a material to deform elastically and return to its original shape upon the removal of a load is referred to as elasticity. This property is fundamental in many engineering applications where materials are subject to forces. Elastic materials absorb energy when deformed and can completely recover their shape, which is characterized by their elastic limit—the maximum stress that can be applied without causing permanent deformation.

In contrast to elasticity, plasticity refers to a material's capacity to undergo permanent deformation. Once a plastic material has been deformed beyond its yield point, it does not return to its original shape. Ductility describes a material's ability to deform under tensile stress, often characterized by the material's ability to be stretched into a wire, but this concept mainly revolves around the extent of plastic deformation rather than elastic behavior. Brittleness indicates a material's tendency to fracture or break under stress without significant deformation, contrary to the ability to return to an original shape.

Thus, elasticity is the defining characteristic of materials capable of returning to their original form after the load is removed, making it the correct choice for this question.