Which anion forms the smallest number of insoluble salts?

${\mathrm{NO}}_3^{-}$ anion forms the smallest number of insoluble salts. Salts that are insoluble in water at normal temperature are referred to as insoluble salts. Because water molecules are unable to draw the ions from the salt complex, they are insoluble in water. As a result, water molecules and insoluble salt compounds do not interact with one another. Hence, option B is correct.