**Core Concept**
Dental caries, also known as tooth decay, is a bacterial infection that causes demineralization of tooth enamel, leading to the formation of cavities. The prevention of dental caries involves reducing the acid production by oral bacteria and promoting the remineralization of tooth enamel.

**Why the Correct Answer is Right**
The correct answer is fluoride, which is a well-established agent for preventing dental caries. Fluoride works by inhibiting the enzyme enolase, which is involved in the glycolytic pathway of oral bacteria, thereby reducing acid production. Additionally, fluoride promotes the remineralization of tooth enamel by forming fluorapatite, a more stable and resistant form of tooth enamel.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because while good oral hygiene practices, such as brushing and flossing, are essential for preventing dental caries, they are not the primary method of prevention. Good oral hygiene can help remove bacteria and food particles that contribute to dental caries, but it does not directly prevent the formation of cavities.
**Option B:** This option is likely incorrect because while calcium and phosphate are essential minerals for maintaining healthy teeth, they do not directly prevent dental caries. While calcium and phosphate can help promote the remineralization of tooth enamel, they do not have a direct effect on reducing acid production by oral bacteria.
**Option C:** This option is likely incorrect because while saliva is essential for maintaining oral health, it is not a direct method of preventing dental caries. Saliva can help neutralize acid and promote the remineralization of tooth enamel, but it is not a primary agent for preventing dental caries.

**Clinical Pearl / High-Yield Fact**
The most effective way to prevent dental caries is through a combination of good oral hygiene practices, fluoride use, and regular dental check-ups. Remember the phrase "F-I-T" to remind yourself of the three key components of dental caries prevention: Fluoride, Intake of a balanced diet, and Tooth brushing (F-I-T).

**Correct Answer:** D. Fluoride