Which fluoride comes in solution form?

Fluorosilic acid is the only option among the choices that is commonly available in a liquid solution form. It is often used in water fluoridation processes. This compound, which is generated as a byproduct from the production of phosphate fertilizers, can be easily dissolved in water and is effective for increasing fluoride concentrations in drinking water.

Sodium fluoride, while also a fluoride compound, is typically available in a solid form and requires dissolution in water before it can be used. Calcium fluoride occurs as a solid mineral and is not readily soluble in water under normal conditions. Stannous fluoride, although it can be formulated in liquid products, is usually found as a powder or in gels, needing specific preparation to create a solution.

Thus, fluorosilic acid's inherent properties allow it to function effectively in water treatment processes that require a fluoride solution.