The correct answer is A) causes corrosion. The presence of air trapped in a non-treated diesel engine jacket water cooling system can lead to corrosion within the system. The air trapped in the cooling system reacts with the water, creating an electrolytic reaction that can corrode the metal components of the cooling system over time. This corrosion can damage the engine block, water pump, and other critical parts, ultimately reducing the ability of the system to effectively cool the engine. The other options are incorrect because they do not fully capture the most serious effect of trapped air in a non-treated cooling system. While options B and C describe potential consequences, they do not identify corrosion as the primary and most serious effect. Option D is incorrect as it does not accurately describe the effect of trapped air on the cooling system.