Correct Answer is: (b) μ > a/h
Let C be the centre of mass of the block. When the force of friction acts on the block, the normal reaction (N) of the ground shifts to the right so as to balance the torque due to F about C. The condition of toppling arises when N reaches the edge of the cube. Taking torque about C, toppling occurs when
F x h/2 > N x a/2
or μmgh > mga or μ > a/h.