The correct answer is D) dissipating the heat of an external flame. Flame screens installed on tank vents that may contain combustible gases are designed to prevent explosions by dissipating the heat of an external flame. These screens act as a barrier to prevent the flame from entering the tank and igniting the flammable vapors inside. By dissipating the heat, the flame screens help ensure that the flammable vapors cannot be ignited, thus preventing the risk of an explosion. The other options are incorrect because they do not accurately describe the primary function of the flame screens. Option A) is incorrect because the screens do not absorb flammable vapors. Option B) is incorrect because the screens prevent the escape of flammable vapors, not allow them to escape. Option C) is incorrect because the screens do not prevent flammable vapors from entering the tank, but rather prevent the ignition of those vapors.