The correct answer is D) You must sound three short blasts on the whistle. This is the correct answer because according to the International Regulations for Preventing Collisions at Sea (COLREGS), when a vessel is taking emergency action to avoid a collision, such as going full speed astern, it must sound three short blasts on the whistle to indicate that it is taking such action. This applies regardless of whether the vessels are meeting, crossing, or in any other relation to each other, as long as they are in sight of one another. The other answer choices are incorrect because A) is only for backing to starboard, B) is incorrect as whistle signals are required in this situation, and C) is incorrect as three short blasts are required when taking emergency action.