The correct answer is D) A rotating impeller and a stationary diffuser. In a centrifugal compressor assembly, the key components are a rotating impeller and a stationary diffuser. The rotating impeller draws in air or gas and accelerates it outwards due to centrifugal force. The stationary diffuser then converts the high-velocity flow from the impeller into increased pressure through controlled deceleration of the flow. The other answer choices are incorrect because they do not accurately describe the core components of a centrifugal compressor. Option A describes a centrifugal fan, not a compressor. Option B describes an axial compressor, and option C describes a positive displacement compressor, which are different types of compressor assemblies.