Which of the following is a key characteristic of potable water tanks?

Potable water tanks are specifically designed to store water that is safe for human consumption, which is why holding drinkable water is their key characteristic. These tanks are constructed to ensure that the water remains uncontaminated and meets health safety standards necessary for public use.

Considering the other options, the first choice describes a function that is contrary to what potable water tanks are intended for. The second option, regarding always being full of liquids, does not accurately define the operational characteristics of these tanks, as they can be partially filled as usage varies. The third option about maintaining high-pressure conditions may apply to certain types of systems or storage methods, but it is not a defining feature of all potable water tanks, which can also operate under varying pressure conditions depending on their design and the needs of the supply system. Thus, the defining characteristic of these tanks is indeed their function of holding drinkable water.