What characteristic describes a material that, when unloaded, retains a permanent elongation?

A material that, when unloaded, retains a permanent elongation is characterized by plasticity. Plasticity refers to the ability of a material to undergo significant deformation without breaking, and this deformation remains even after the removal of the stress that caused it. In practical terms, this means that once the material has been stretched or deformed beyond its elastic limit, it does not return to its original shape; instead, it retains a new, permanent shape or size.

In contrast, elasticity describes a material's ability to deform under stress and then return to its original shape once that stress is removed. Ductility refers specifically to the ability of a material to be drawn out into a wire, which is a specific type of plastic behavior under tensile stress but does not cover all aspects of maintaining a permanent elongation. Toughness is the ability of a material to absorb energy and plastically deform without fracturing, which involves both strength and ductility but doesn't necessarily relate to retaining permanent elongation under unloading conditions.

Therefore, the defining characteristic of a material that retains permanent elongation upon unloading aligns with plasticity, making it the correct choice.