(i), (ii), (iii), (iv) and (vi).

The distance between two masses in a system is inversely proportional to their potential energy. As the two balls grow closer to each other, the potential energy of the system will decrease. When the balls collide, the potential energy is zero, which occurs at r = 2R. These conditions are not met by the potential energy curves in I (ii), (iii), (iv), and (vi). As a result, there isn’t any elastic collision.