Fluoride is added to drinking water in the form of:: (A) Hydroﬂuosilicic acid, (B) Fluoride ions, (C) APF, (D) None of the above
Answer is A. Fluoride compounds used in water fluoridation

Fluorspar 
Sodium fluoride 
Silicofluoride 
Sodium silicofluoride 
Ammonium silicofluoride 
Hydrofluosilicic acid.

Essentials of preventive and community dentistry
Soben Peter page no. 268