Final answer:
All the compounds listed, including Sodium fluoride (NaF), stannous fluoride (SnF2), and sodium monofluorophosphate (Na2FPO3), have been active ingredients in fluoride toothpaste; hence, the answer is 'None of the above'.
Step-by-step explanation:
The student asked which of the options listed has never been an active ingredient in fluoride dentifrice. Sodium fluoride (NaF), stannous fluoride (SnF2), and sodium monofluorophosphate (Na2FPO3) have all been used as active ingredients in fluoride dentifrice. Therefore, the answer to the question is D. None of the above. The use of these compounds in toothpaste is significant due to their role in preventing tooth decay by strengthening tooth enamel and providing some antimicrobial effects to reduce the incidence of dental caries.