In the relationship between the restoring force exerted by an ideal spring and its displacement from equilibrium, the constant k most nearly represents which of the following physical quantities?
a) Half the slope of a graph of elastic potential energy U versus x2, in N/m.
b) The negative slope of the graph of restoring force F versus displacement x, in N/m.
c) The stiffness of the spring, defined as the restoring force per unit displacement, measured in N/m.
d) The ratio of the maximum restoring force the spring can exert to its natural length, in N/m.