The correct answer is D) rack position. The amount of fuel delivered by a unit injector is controlled by the rack position, which is the position of the fuel metering rack in the injector. The rack position determines the amount of fuel that is injected into the engine cylinder during each combustion cycle. This allows the engine's fuel delivery to be adjusted based on the operational demands, such as engine speed and load. The other answer choices are incorrect because they do not directly control the fuel delivery of the unit injector. Engine speed, the main spring, and the camshaft may influence other aspects of the engine's operation, but they do not directly regulate the amount of fuel injected by the unit injector.