The correct answer is C) Specific gravity of the fuel. The size of the ring dam for a fuel oil centrifugal purifier is determined by the specific gravity of the fuel being purified. The ring dam acts as a barrier to separate the fuel from the contaminants, and its size needs to be adjusted based on the density or specific gravity of the fuel to ensure proper separation. Fuels with higher specific gravities require a larger ring dam to effectively remove water and other impurities. The other factors, such as viscosity, quantity of water, and quantity of dirt, can influence the purifier's overall performance, but the specific gravity is the primary factor that determines the appropriate size of the ring dam.