The correct answer is B) With flame screens. When a cargo tank's ullage opening is open and the tank is not gas free, the tank's atmosphere can contain flammable vapors. To prevent these vapors from igniting, the regulations require the use of flame screens over the ullage opening. Flame screens act as a barrier to prevent any external ignition source from entering the tank and igniting the flammable vapors. The other options, such as pressure-vacuum relief valves, warning signs, or gooseneck vents, do not provide the necessary physical barrier to prevent ignition that flame screens provide in this situation.