The correct answer is B) Suction lines are fitted with a non-return valve. The purpose of a non-return valve on the suction lines of double bottom or peak tanks is to prevent accidental flooding in the event of a pipe rupture due to collision. The non-return valve allows fluid to flow in only one direction, preventing backflow and unintended flooding of the tanks. The other answer choices are incorrect because: A) Reducing the number of pipes does not prevent flooding if a rupture occurs. C) Separate fill and discharge lines do not inherently prevent flooding. D) Vertical pipe routing above the margin line does not automatically stop backflow.