A stable system is the one which could resist the disturbance to maximum extent. If we look at the box A, a little disturbance can change its position because the area of contact between the box and the surface is very small which means the centre of mass is not at its correct position. It will rotate to left or right if pushed slightly and it can become stable when it resist more disturbance. In simpler words if box A is disturbed in any means it will gain the position of box B. So box B s more stable because any disturbance will not change its position
