The correct answer is B) not impede the passage of vessel "B". The reasoning behind this is that under the International Regulations for Preventing Collisions at Sea (COLREGS), vessels engaged in fishing are considered to be the give-way vessel when meeting a power-driven vessel (like the tanker, vessel "B") in a narrow channel. Therefore, vessel "A" should take action to not impede the safe passage of the tanker (vessel "B"), which is sounding the five short and rapid blasts to indicate that it is taking action to avoid a collision. The other answer choices are incorrect because: A) sounding whistle signals is not the appropriate action for vessel "A" to take; C) not answering the whistle signals would be improper and could be unsafe; and D) maintaining course and speed would impede the passage of the give-way vessel (vessel "B"), which is not allowed under the COLREGS.