The correct answer is D) if the cargo has been discharged symmetrically it is an indication of negative GM. When a Ro Ro vessel discharges its cargo symmetrically, meaning the cargo has been removed evenly from both sides of the vessel, and the vessel then develops a list, it is an indication that the vessel has a negative GM (Metacentric Height). Negative GM means the vessel's center of gravity is above the metacenter, making the vessel unstable and prone to listing. This could be caused by improper cargo discharge or loading procedures that result in an uneven distribution of weight on the vessel. The other answer choices are incorrect. Option A is incorrect because sewage discharge would not directly cause a list. Option B is incorrect because self-tension winches would not be the cause of a list in this scenario. Option C is incorrect because a positive GM (high center of gravity) would not cause a list if the cargo has been discharged symmetrically.