The correct answer is A) improper adjustment of the bypass valve. In a common rail fuel system, the bypass valve is responsible for controlling the pressure in the fuel header. If the bypass valve is not properly adjusted, it can lead to excessive pressure building up in the header. This is because the bypass valve is not able to effectively release the excess pressure, causing it to accumulate in the header. The other answer choices are incorrect because they do not directly relate to the issue of excessive pressure in the header. A dribble in the fuel injection nozzle, insufficient leakoff through the injection nozzle packing, or a malfunctioning injection nozzle would not cause the excessive pressure in the header that the question is asking about.