A horizontal plank is executing SHM along the vertical direction with angular frequency $\omega .$ A coin is placed on top of this plank. If the amplitude of oscillation is increased gradually, for what maximum amplitude will the coin be on the verge of loosing contact with the plank?
(A) When is plank is at its maximum height
(B) When the plank is at the midpoint.
(C) When the amplitude is $\left(\mathrm{g} / \omega^{2}\right)$
(D) When the amplitude is $\left(\mathrm{g}^{2} / \omega^{2}\right)$