The correct answer is E. That boulder's potential energy is just waiting to become kinetic, but physics has a dark sense of humor. When E pushes the stone, it'll roll down, hit the see-saw at C, launch the spear into D, who will fall into the pit. Meanwhile, B keeps walking obliviously, and A stands there contemplating their life choices. But E? Poor E will be crushed by that suspended half-sphere above them that nobody noticed. Conservation of energy, conservation of life expectancy... not so much.