A ball is dropped on a floor and bounces back to a height somewhat less than the original height. Which of the curves depicts its motion correctly?

When a ball is dropped on a floor,

$y=\frac{1}{2}g{t}^2$                                             …..(1)

So the graph between y and t is a parabola. Here as time increases, y decreases. 

When the ball bounces back, then

$y=ut+\frac{1}{2}g{t}^2$                                      …..(2)

The graph between y and t will be a parabola. But here as time increases, y also increases. So (b) represents the graph.