Concept:An object is in neutral equilibrium when a slight displacement neither increases nor decreases its potential energy, so it stays in its new position.
Explanation:From the diagram, ball Q rests on top of a curved crest, so a small push makes it roll down and not return. This is unstable equilibrium.
Ball R sits at the bottom of a valley, so a small push makes it roll back to the same point. This is stable equilibrium.
Ball S lies on a flat, horizontal portion of the ground. If displaced a little, it has no tendency to return or to move further; it simply balances at the new position.
Thus, only ball S is in neutral equilibrium.
Answer:A. S only
