The correct answer is C) lack of moving parts. The key distinguishing feature of an eductor, also known as a jet pump or ejector, is that it has no moving parts. Unlike other types of pumps that rely on an impeller or other mechanical components to generate flow, an eductor uses the Venturi effect to create suction and move fluid. This lack of moving parts makes the eductor a simple, low-maintenance, and often more reliable pump design compared to other pump types. The other answer choices are incorrect because they do not accurately describe the defining characteristics of an eductor. The size of the impeller is not a distinguishing feature, the ease of changing wearing rings is not unique to the eductor design, and the fact that the discharge end is smaller than the suction end is a general characteristic of Venturi-based pumps, not specifically the eductor.