The correct answer is D) Heat transfer always flows from hot regions to cold regions. This is correct because the second law of thermodynamics states that heat naturally flows from hot regions to cold regions, and not the other way around. Heat will always spontaneously flow from a hotter object to a cooler object, never the reverse, unless external work is done to force the heat to flow in the opposite direction. The other options are incorrect because A) heat does not always flow from cold to hot, B) gases can sometimes transfer heat less efficiently than liquids, and C) the material of the pipe does not determine the direction of heat transfer, only the efficiency.