Which of the following describes stiffness in materials?

The concept of stiffness in materials refers specifically to the resistance a material exhibits when subjected to deformation under load. Stiffness is quantitatively defined as the ratio of the applied load to the resulting displacement or deformation of the material. This makes stiffness crucial in applications where maintaining a structure's shape and integrity under force is important.

The statement regarding the ability to return to original shape describes elasticity, which is a different material property. Similarly, the resistance to penetration speaks to hardness or toughness, which are also distinct from stiffness. Therefore, the characterization of stiffness as a measurement of deformation under load accurately captures its essence and is foundational in materials science and engineering.