ANS:D - load to produce unit deflection.

The stiffness of a spring is measured by the load to produce unit deflection. Stiffness refers to the resistance of a spring to deformation when subjected to an applied load. It is commonly measured by determining the amount of force (load) required to produce a specific amount of deflection (displacement) in the spring. The load required to produce a unit deflection, such as force per unit length or force per unit displacement, is a measure of the spring's stiffness. The ability of a spring to absorb shock or its capacity to store energy are related concepts but are not direct measures of stiffness. Similarly, the ratio of the wire and coil diameters may affect the mechanical properties of the spring but does not directly measure stiffness. Therefore, the correct measure of stiffness is the load to produce unit deflection.