The correct answer is C) cause the unit to flop to a greater angle. When a vessel experiences a sudden severe list due to negative initial stability, counter-flooding into empty ballast tanks can actually make the situation worse. Negative initial stability means the vessel is inherently unstable and will continue to list further when a disturbance occurs. Counter-flooding the empty ballast tanks in this case will shift the center of gravity even further from the upright position, causing the vessel to "flop" to a greater angle of list rather than returning it to an upright equilibrium. The other answer choices are incorrect because: A) increasing the righting arm would require positive initial stability, which is the opposite of the given scenario; B) bringing the unit to an upright equilibrium is the desired outcome, but not the result of counter-flooding with negative initial stability; and D) increasing the righting moment would also require positive initial stability, which is not the case here.