Answer:
c. At their highest point, both stones will have the same gravitational potential energy because they reach the same height.
Explanation:
The above statement is true going by the fact that, the gravitational potential energy of both stones is a function of its mass (weight) as well as the height at which it reach. Despite the fact that, both stones has different weights, it would reach different heights. At their various maximum height, the will have same potential energy.