(a) In the first case [Figure (i)],the direction of force F and the direction of displacement are at right angles to each other. So, the work done in the first case is zero.
(b) In the second case [Figure (ii)], the displacement is in the direction of force F, so the work done is positive.
(c) In the third case [Figure (iii)], the force F acts in a direction opposite to the direction of displacement, so the work done is negative.