The correct answer is C) improper adjustment of the bypass valve. In a common rail fuel system, the bypass valve is responsible for regulating the pressure in the fuel header. If the bypass valve is not properly adjusted, it can cause excessive pressure buildup in the header. This is because the bypass valve is meant to divert excess fuel back to the fuel tank when the pressure reaches a certain threshold. Improper adjustment of this valve can prevent it from doing its job effectively, leading to the observed issue of excessive pressure. The other options are incorrect because they do not directly address the root cause of excessive pressure in the header. A malfunctioning injection nozzle (A) or insufficient leakoff through the nozzle packing (B) may contribute to pressure issues, but they do not directly cause the excessive pressure in the header. A dribble in the fuel injection nozzle (D) is also not the primary cause of the excessive pressure.