(i) in (a) the direction of force, F and displacment are perpendicular to each other. There is no displacement in the direction of force and as such work done is zero.
(ii) (b) the direction of force F and displacement are in the some direction and as such work done by the force is positive.
(iii) (c ), the force, F acts in a direction opposite to the displacement and as such work done is negative.