Two balls are dropped from the top of a high tower with a time interval of t0 second, where t0 is smaller than the time taken by the first ball to reach the floor, which is perfectly inelastic. The distance S between the two balls, plotted against the time lapse / from the instant of dropping the second ball, is best represented by

Before the second ball is dropped, the first ball would have travelled some distance say S0=12gt02. After dropping the z second ball, the relative acceleration of both balls becomes zero. So distances between them increase linearly. After some time, the first ball will collide with the ground and the distance between them will start decreasing and the magnitude of relative velocity will be increasing for this time. Option (d) represents all these clearly.