The correct answer is D) lack of moving parts. An eductor, also known as a jet pump or ejector, is a type of pump that operates without any moving parts. Instead, it uses the Venturi effect to create suction and move fluid. The lack of moving parts is a distinguishing feature of an eductor compared to other types of pumps, which typically have impellers, vanes, or other moving components that drive the fluid flow. The other answer choices are incorrect because they do not accurately describe the distinguishing features of an eductor. The discharge end being smaller than the suction end (A) is a characteristic of the Venturi effect, not a distinguishing feature. The ease of changing wearing rings (B) and the small size of the impeller (C) are not unique to eductors.