The correct answer is A) 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. The RO membrane can allow some bacteria and viruses to pass through, which could potentially contaminate the potable water. To ensure the water is safe for consumption, a sterilizer such as a chlorinator or ultraviolet light system must be used in conjunction with the RO unit. The other options (B, C, and D) do not necessarily require a sterilizer, as the titanium plate, submerged tube, and multi-stage flash desalination processes may effectively remove microorganisms without the need for additional sterilization, depending on the specific design and water quality requirements.