The correct answer is B) Downslope motion of an air mass along a coast. Downslope motion of an air mass along a coast can act to dissipate fog by warming and drying the air, causing the fog to evaporate. As the air descends the slope, it undergoes adiabatic warming, which reduces the relative humidity and causes the fog to dissipate. The other options are incorrect because: A) Upwelling of cold water would actually tend to increase fog formation by cooling the air. C) Advection of warm air over a colder surface can lead to fog formation, not dissipation. D) Warmer rain would not necessarily dissipate fog, as the fog may still persist in the cooler air.