The correct answer is A. Stiffness.
Stiffness is a measure of how difficult it is to deform an object. It is calculated by dividing the force applied to an object by the resulting deformation. In the case of a spring, the stiffness is the force required to stretch or compress the spring by a unit distance.
Proof resilience is the ability of a material to withstand repeated deformation without permanent damage. Proof stress is the maximum stress that a material can withstand without permanent deformation. Proof load is the load that causes a material to reach its proof stress.
Stiffness is important in many engineering applications, such as designing bridges and buildings. A bridge that is too flexible will sway in the wind, making it uncomfortable for people to cross. A building that is too flexible may collapse in an earthquake. By choosing materials with the right stiffness, engineers can design structures that are strong and stable.