The work done is equal to the dot product of force times displacement. If the force is perpendicular to the direction of displacement, then the work done is zero. In elliptical orbits, this situation occurs at the apogee and perigee points. Hence, option (b) is true and (d) is false. In rest of the points, the force makes an angle with displacement and hence, work done is not zero. Therefore, (a) is false. In one complete revolution, the total displacement is zero. Hence the work done is zero and (c) is true.