If the ball hits the $n^{th}$ step vertical distance travelled in $nh$ horizontal distance travelled is $nb$
Let $t$ be the time of fall
Initaial vertical velocity $=0$
Therefore $nb=ut$----(1)
$nh=0+\large\frac{1}{2}$$gt^2$-----(2)
from (1) and (2)
$n= \large\frac{2hu^2}{gb^2}$
Hence d is the correct answer.