Answer
A, B, C
Work Step by Step
Remember that the spring constant is a measure of stiffness. It is the ratio of force to distance. Since the distance is the same in each graph, the force on the box is related to the spring constant. The largest kinetic energy curve will provide the most force on the box, and therefore have the greatest spring constant.
We can see from the graph that the largest curve is A, followed by B, and then C.
