Which form of fluoride is classified as liquid fluoride?

Hydrofluorosilicic acid is classified as a liquid fluoride because it is typically used in its aqueous solution form, making it readily available for water treatment applications. This compound is commonly utilized in water fluoridation processes to help prevent dental caries, as it effectively increases the fluoride concentration in drinking water.

In contrast, the other options such as sodium fluoride, calcium fluoride, and potassium fluoride are generally encountered in solid or crystalline forms. While these compounds can also provide fluoride ions, they are not liquids in their standard state and would need to be dissolved in water to be used effectively in processes like water fluoridation. Therefore, hydrofluorosilicic acid stands out as the option that is inherently a liquid.