Which of the following components could be fitted on a diesel engine?

A de-compressor valve is a component that can be fitted on a diesel engine to help with starting the engine, especially in cold conditions or when the engine is under heavy load. This valve allows air to escape from the cylinder during the compression stroke, reducing the compression pressure during starting. As a result, it decreases the effort required from the starter motor, making it easier to start the engine.

In contrast, while the other components mentioned may also be used in specific contexts within diesel engines or other types of engines, they do not exclusively pertain to diesel engine functionality or are not as directly related to the operation or maintenance of a diesel engine. A supercharger, for example, compresses the intake air to boost power, but its application is less common in diesel engines compared to turbocharging due to the inherent characteristics of diesel combustion. A turbine typically refers to a component used in turbochargers which assists in increasing the engine's efficiency, but it is not an independent fitting on the engine itself. Meanwhile, a variable intake manifold can be an advanced feature in some modern engines to optimize performance, but its usage is more common in gasoline engines than in traditional diesel engines.