The correct answer is C) Air flow through the radiator. The proper operation of a radiator-cooled engine is essential for maintaining the engine's temperature within the optimal range. The key factor is the air flow through the radiator, as this air flow is responsible for dissipating the heat generated by the engine's combustion process. Without adequate air flow, the radiator cannot effectively cool the engine, leading to overheating and potential damage. The other options are not directly related to the proper operation of a radiator-cooled engine. Cooling water pressure (A) is important for the circulation of the coolant, but it is not the essential factor. Low heat of combustion (B) is a property of the fuel, not a factor in the cooling system. Jacket water treatment (D) is important for maintaining the coolant quality, but it is not the primary factor for the radiator's cooling performance.