The correct answer is D. 04 kg/cm2 of FP pressure in the engine.
The FP pressure is the minimum pressure required to operate the air brakes on a train. The higher the FP pressure, the more powerful the brakes will be. For a 40 BCN air braked train, the FP pressure must be at least 04 kg/cm2. This is to ensure that the brakes are able to stop the train safely in an emergency.
The other options are incorrect because they are not high enough to operate the air brakes on a 40 BCN air braked train. Option A, 5 kg/cm2, is the minimum pressure required to operate the air brakes on a 30 BCN air braked train. Option B, 02 kg/cm2, is the minimum pressure required to operate the air brakes on a 20 BCN air braked train. Option C, 03 kg/cm2, is the minimum pressure required to operate the air brakes on a 10 BCN air braked train.
It is important to note that the FP pressure must be maintained at a constant level throughout the journey. If the pressure drops below the minimum required level, the brakes will not be able to operate effectively and the train may not be able to stop safely.