d. Just before it is released, the box has twice as much elastic potential energy as before
When a spring is compressed, it stores potential energy in the form of elastic potential energy. The amount of elastic potential energy stored in the spring is given by the formula:
E = 1/2 * k * x^2
where E is the elastic potential energy, k is the force constant of the spring, and x is the distance the spring is compressed.
Therefore, if we compress the spring at a distance of 2x instead of x, the elastic potential energy stored in the spring will be twice as much. This means that just before the box is released, it has twice as much elastic potential energy as before.