The correct answer is C) at low vessel speeds. The reason for this is that at low vessel speeds, a current perpendicular to the vessel's track has a greater effect on the vessel's course made good. This is because at low speeds, the vessel has less momentum to overcome the deflecting effects of the current, causing a more significant deviation from the intended course. In contrast, at high speeds, the vessel's momentum is greater and can better overcome the influence of the perpendicular current. The other options are incorrect because vessel speed, not water depth, is the key factor in determining the effect of a perpendicular current on the course made good. Deep or shallow water does not have a significant impact on this relationship.