The correct answer is A) cylinder air starting check valves. During the starting of a diesel engine, the cylinder air starting check valves prevent compression gases from backing into the air starting system. These check valves are one-way valves that allow air to flow from the air starting system into the engine cylinders, but not in the reverse direction. This prevents the high-pressure compression gases in the cylinders from being forced back into the air starting system, which could potentially damage the components in the air starting system. The other answer choices are incorrect because: B) individual distribution valves are not responsible for preventing backflow, C) the high pressure in the starting air manifold alone does not prevent backflow, and D) the air starting control valve controls the starting air supply but does not directly prevent backflow.