The correct answer is D) A serious hazard of blood poisoning exists if the fuel spray penetrates the skin of the operator. This is correct because diesel fuel injection nozzles can produce a fine, high-pressure spray that can easily penetrate the skin. If this happens, the fuel can be absorbed into the bloodstream, leading to a serious and potentially life-threatening condition called diesel fuel poisoning or fuel sepsis. The other answer choices are incorrect because: A) The opening pressure is typically around 75-100% of the popping pressure, not 50%. B) The needle valve spring does not need to be removed for basic nozzle testing. C) The needle valve should close when the nozzle pops open, not remain open.