The correct answer is A) prevent flames from entering tanks. Flame screens are used to prevent flames from entering tanks, particularly fuel tanks, in order to avoid the risk of explosions or fires. Regulations require the use of flame screens on tank vents and other openings to combustible spaces to contain any flames or sparks and prevent them from reaching the potentially flammable vapors inside the tanks. The other options are incorrect because: B) flame screens are not primarily used to protect firefighters, C) they are not specifically for containing engine exhaust, and D) while they do help contain flammable fumes, their primary purpose is to prevent flames from entering the tanks themselves.