The correct answer is D) permits the passage of vapor but not of flame. A flame screen is a device that allows the passage of vapors but prevents the passage of flame. It is designed to prevent the transmission of a flame from one side of the screen to the other, acting as a barrier to stop the spread of fire. This is an important safety feature for tanks or other enclosed spaces that may contain flammable vapors. The other answer choices are incorrect because they do not accurately describe the function of a flame screen. Option A) does not address the prevention of flame passage, option B) is too broad, and option C) is specific to inert gas rather than vapors.