The correct answer is D) specific gravity of the oil being purified. The size of the discharge ring in a disk-type purifier is dependent on the specific gravity of the oil being purified. This is because the discharge ring controls the flow of the oil through the purifier, and the specific gravity of the oil affects how it behaves and flows through the system. The specific gravity, not the viscosity, rated capacity, or maximum design speed, is the critical factor in determining the appropriate discharge ring size for efficient purifier operation. The other options are incorrect because they do not directly affect the discharge ring size in the same way as the specific gravity of the oil.