The correct answer is C) It decreases at increased angles of heel due to pocketing when a tank is 90% full. The free surface effect refers to the reduction in a vessel's stability caused by the movement of liquid in partially filled tanks. When a tank is 90% full, the liquid will "pocket" or collect at the ends of the tank as the vessel heels, decreasing the free surface effect and the destabilizing influence of the liquid. As the angle of heel increases, the free surface effect will decrease due to this pocketing phenomenon. The other answer choices are incorrect: A) This is incorrect as the free surface effect is a real reduction in stability, not a virtual reduction. B) This is incorrect as the free surface effect is not directly proportional to the specific gravity of the liquid. D) This is incorrect as the free surface effect is not directly proportional to the length and breadth of the tank.