Answer: d. an energy transformation.
Step-by-step explanation:
Gravitational potential energy is the energy possessed by an object by virtue of its position.
Work is defined as the force leading to displacement of the point of application in the direction of the force.
Law of conservation of energy, energy can neither be created nor be destroyed. It can only transformed from one form to another.
Energy transformation occurs when one form of energy is converted into another form. For example : A body in rest possess potential energy and when it moves potential energy is converted into kinetic energy.