The correct answer is B) loosening the compression nuts at the injectors. The reason this is the correct answer is that air is bled from a diesel engine fuel system by loosening the compression nuts at the injectors. This allows any air bubbles or pockets to be released from the fuel lines and injectors, ensuring that the fuel system is fully primed and free of air. This is a standard procedure for bleeding air from a diesel fuel system. The other answer choices are incorrect because: A) blowing down the air tanks would not bleed air from the fuel system, C) changing fuel filters would not directly bleed air, and D) pumping down the day tanks would not release air from the injectors.