A block is resting over a smooth horizontal plane. A constant horizontal force starts acting on it at t = 0. Which of the following graphs between acceleration and time is correct ?

When a block is placed on a smooth horizontal surface, the surface offers normal reaction. The normal reaction balances the weight of the block. When a constant horizontal force is applied on the block, it starts accelerating along the direction of force with constant acceleration F/m. So, the graph between a and t will be a straight line parallel to time axis.