What property measures the ability to resist deformation under stress?

Stiffness is the property that specifically measures a material's ability to resist deformation when subjected to stress. It quantifies how much a material will deform under a given load, making it crucial in applications where maintaining shape and form is essential. Stiffness is often defined as the ratio of the applied force to the resulting displacement, meaning the higher the stiffness, the less a material will deform.

Plasticity refers to a material's ability to undergo permanent deformation without breaking when stress is applied. Toughness is a measure of a material's ability to absorb energy and plastically deform without fracturing, indicating both strength and ductility.

While each of the other properties relates to how materials behave under stress, stiffness uniquely characterizes the resistance to elastic deformation. Thus, stiffness is the correct choice when identifying a material's resistance to deformation.