●A needle valve is a type of valve with a small port and a threaded, needle-shaped plunger. It allows precise regulation of flow, although it is generally only capable of relatively low flow rates. Needle valve regulates the pressure strokes in the fuel injection pump of a Diesel engine.

●The purpose of the fuel injection system is to deliver fuel into the engine cylinders, while precisely controlling the injection timing, fuel atomization, and other parameters. The main types of injection systems include,pump-line-nozzle, unit injector, and common rail.

Hence, the correct option is (D).