The correct answer is B) domestic water tank. Potable (drinkable) water systems on vessels are typically connected directly to the domestic water tank, which is a dedicated freshwater storage tank used for drinking, cooking, and sanitary purposes. This arrangement ensures that the potable water supply is kept separate from other water systems on the vessel, such as the fire-main system or feed-water system, to maintain the quality and safety of the drinking water. The other options are incorrect because: A) the freshwater sanitary system is not the primary source of potable water; C) the fire-main system is used for firefighting purposes and should not be connected to the potable water system; and D) the feed-water system is used to supply water to the vessel's boilers or other machinery, which is not suitable for direct potable water use.