The correct answer is D) permits the passage of vapor but not of flame. The primary function of a flame screen on a tank vessel is to prevent the passage of flame into the tank while still allowing the passage of vapor. This is important to prevent the ignition of flammable vapors inside the tank. Flame screens are required by regulations to be installed on tank openings and vents to serve this purpose. The other options are incorrect because they do not accurately describe the primary function of a flame screen. Option A is incorrect as the flame screen does not prevent the passage of flammable vapors, only the passage of flame. Option B is incorrect as the flame screen does not selectively permit vapors to exit but not enter a tank. Option C is incorrect as the flame screen is not used to prevent the escape of inert gas from a tank.