The correct answer is C) go to port. When a twin-screw vessel is moving astern with rudders amidships, and the starboard screw suddenly stops turning, the vessel's head will go to port. This is because the loss of thrust on the starboard side will cause the vessel to turn towards the side with the still-functioning screw, which is the port side. The difference in thrust between the two screws will create a yawing moment that will cause the vessel to turn to port. The other answer choices are incorrect because: A) the vessel would turn to port, not starboard, B) the vessel would not suddenly drop down, and D) the vessel would not remain stationary, as the loss of thrust on the starboard side would cause it to turn.