What best describes the ability of a material to deform under tensile stress?

The ability of a material to deform under tensile stress is best described by ductility. Ductility refers specifically to a material's capacity to undergo significant plastic deformation before rupture or failure. This property allows materials to stretch and shape without breaking when subjected to tensile forces, making ductile materials particularly useful in applications such as welding and metal forming, where shaping materials is essential.

In contrast, elasticity is about a material's ability to return to its original shape after the stress is removed. While resilience denotes a material's ability to absorb energy and deform elastically, it does not imply the capability to sustain large deformations. Toughness refers to a material's ability to absorb energy and plastically deform without breaking, encompassing both strength and ductility but is a broader concept that includes resistance to fracture.

Therefore, when specifically focusing on deformation under tensile stress and the ability to extend significantly before failure, ductility is the most accurate descriptor.