Stiffness is defined as the ability of a material to:

Stiffness refers to a material's resistance to deformation when subjected to an external force or load. When a load is applied to a material, a stiff material will maintain its shape and size, exhibiting minimal deformation. This characteristic is vital in engineering and construction, as it ensures that structures and components can bear loads without experiencing significant changes in their dimensions.

Other definitions presented in the options focus on different aspects of material behavior. For instance, resisting penetration pertains more to hardness than stiffness. Retaining a deformed shape when the load is removed describes plastic deformation, which is not related to stiffness. The ability to return to the original shape after deformation is known as elasticity, not stiffness. Thus, the choice that correctly defines stiffness is the one that focuses on resistance to changes in shape and size under load.