Concept:Toothpaste prevents tooth decay by neutralizing the acid produced by bacteria in the mouth.
Explanation:Bacteria in the mouth break down sugar and food particles, producing acid.
This acid attacks and destroys the tooth enamel, causing decay.
Toothpastes are basic in nature.
They neutralize the excess acidity, protecting the teeth from decay.
The other options are not the primary mechanism: emulsification is not directly involved, fluoride action is mainly topical (remineralization and enzyme inhibition), and a calcium coat is not formed.
Answer:A. neutralizing the excess acidity.