**Core Concept**
Fluoride's primary mechanism of action on topical application is related to its ability to inhibit the enzyme acid phosphatase, which plays a key role in the demineralization of tooth enamel. This process is crucial in preventing tooth decay and promoting oral health.

**Why the Correct Answer is Right**
When fluoride is applied topically, it forms a protective barrier on the tooth surface, reducing the activity of acid phosphatase and other enzymes responsible for demineralization. This action helps to prevent the dissolution of calcium and phosphate ions from the tooth enamel, thereby strengthening the tooth structure and making it more resistant to decay. The inhibition of acid phosphatase is a key factor in fluoride's ability to prevent tooth demineralization.

**Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because fluoride's primary mechanism of action is not related to its antibacterial properties, although it does have some antimicrobial effects.
- **Option B:** This option is incorrect because fluoride does not primarily work through the formation of fluorapatite, which is a secondary effect of its long-term use.
- **Option C:** This option is incorrect because fluoride's primary mechanism of action is not related to its ability to stimulate saliva production, which is a secondary effect of its use.

**Clinical Pearl / High-Yield Fact**
A memorable fact about fluoride is that it is most effective in preventing tooth decay when used in a concentration of 1000-1500 parts per million (ppm) in community water fluoridation systems.

**Correct Answer: D. Inhibition of acid phosphatase enzyme.**