In an elastic collision between two billiard balls. The variation of potential energy is represented by the graph (here r is the distance between centres of the balls and R is radius of the ball)

• A system of two masses has a potential energy that is inversely proportional to their distance from one another. 
• Balls deform after collisions, and the elastic potential energy they contain steadily increases. 
• When deformation is at its maximum, the elastic potential energy also reaches its maximum. 
• Following then, the balls begin to take on their original size and shape, and the potential energy they have accumulated steadily diminishes until it is zero.

Hence the correct answer is option D.