The correct answer is D) Reverse osmosis type unit. Reverse osmosis (RO) desalination plants require a sterilizer when providing water to a potable water system because the RO process alone does not effectively remove all microorganisms from the water. RO membranes have pore sizes that can allow some bacteria and viruses to pass through, so a disinfection step is necessary to ensure the water is safe for human consumption. The other desalination methods (titanium plate, multi-stage flash, and submerged tube) typically have more effective mechanisms for removing microorganisms, so a sterilizer may not be required in those cases.