The right answer is (b). Essentially to increase fuel efficiency one has to avoid as much as possible to accelerate (since to accelerate you have to use more fuel) and to break (because you are then forced to accelerate to regain speed).
If one maintains a good following distance from the preceding vehicle, there is more time to absorb the preceding vehicle variation in speed. In practice one can maintain a constant speed even if the other vehicle is briefly accelerating or using the break. On the contrary, if you stay to near, you have to continuously adjust your speed and that waste fuel.
