Explanation

(b) If the surface is smooth then the kinetic energy at $B$ never be zero
If the surface is rough, the kinetic energy at $B$ be zero. Because, work done by force of friction is negative. If work done by friction is equal to $mgh$ then, net work done on body will be zero. Hence, net change in kinetic energy is zero. Hence, (b) is correct
If the surface is rough, the kinetic energy at $B$ must be lesser than $mgh$. If surface is smooth, the kinetic energy at $B$ is equal to $mgh$
The reason is same as in (a) and (b)