A. potential to kinetic
When the rock was ready to fall off the ledge with energy building up (potential energy), it started falling off the ledge. (kinetic).
Thermal energy comes from heat, generating movement. (No heat in this scenario)
Mechanical energy comes from the sum of kinetic and potential energy in an object that is used to do work. (Like a wrecking ball, not in this case though, because the rock came from stop, to motion, to finish).
Hope this helps :) Brainliest?
Potential energy is converted into kinetic energy. I am 100% positive.