The correct answer is D) Blue. On a weather map, a cold front is typically represented by a solid blue line. This is in accordance with the standard weather map conventions used by the National Weather Service and other meteorological agencies. The blue color signifies the presence of a cold front, which is a boundary between warm and cold air masses where the colder air is advancing and displacing the warmer air. The other answer choices are incorrect because: A) Alternating red and blue lines represent a warm front, not a cold front; B) Red lines represent a warm front; and C) Purple is not a standard color used to depict cold fronts on weather maps.