The correct answer is B) red. The color of the signal flare sent up by a submarine about to surface due to an emergency condition is red. This is because the international regulations for maritime signaling require that a red flare or other red distress signal be used to indicate a vessel in distress and in need of immediate assistance. Red flares are the recognized standard emergency signal for vessels, including submarines, experiencing an emergency situation. The other answer choices (green, white, yellow) are incorrect because they do not correspond to the established emergency signaling protocols for a submerging or surfacing submarine in distress.