The correct answer is C) ignores the effect of surface currents. A dead reckoning (DR) plot is the process of estimating a vessel's position based on its course and speed over a given period of time, without taking into account the effects of external factors like wind and currents. This means that a DR plot does not account for the effect of surface currents, which can significantly impact the vessel's actual position. Other options are incorrect because: A) a DR plot can be started from an assumed position, B) a DR plot is most useful when the vessel's position is unknown, and D) a DR plot uses compass courses rather than magnetic courses.