Answer: Potential energy into kinetic energy.
Explanation: When the rock is in the edge, it is not actually moving, so it only has potential energy U = m*g*h (where m is the mass of the rock, g is the gravity's acceleration and h is the height at which the rock is, in this case, the height of the edge with respect to the ground).
Now, when the rock starts to fall, now the rock has velocity, so it has kinetic energy, which is written as K = (m/2)*v^2 where v is the velocity.
You can see that as the rock starts to fall, the height decreases, and also the potential energy. This is because the potential energy is being converted into kinetic energy, and this is why the velocity of the rock is increasing.
