**Core Concept**
The cariostatic level of fluoride in drinking water refers to the optimal concentration that prevents tooth decay while minimizing the risk of dental fluorosis. This concept is essential in public health dentistry, as it guides the establishment of community water fluoridation programs.

**Why the Correct Answer is Right**
The cariostatic level of fluoride in drinking water is 1 part per million (ppm). This concentration is sufficient to provide a systemic dose of fluoride, which is then incorporated into tooth enamel, making it more resistant to acid attacks from plaque bacteria. The optimal level of 1 ppm is based on extensive epidemiological studies, which have shown that this concentration is associated with the lowest rates of tooth decay and dental fluorosis.

**Why Each Wrong Option is Incorrect**
**Option B:** 1.5 ppm is higher than the optimal level and may increase the risk of dental fluorosis, particularly in children with developing teeth.
**Option C:** 2 ppm is significantly higher than the recommended level and is associated with a higher risk of dental fluorosis and skeletal fluorosis.
**Option D:** 2.5 ppm is even higher than 2 ppm and is not considered a safe or effective concentration for community water fluoridation.

**Clinical Pearl / High-Yield Fact**
The optimal level of fluoride in drinking water is 1 ppm, and this concentration is associated with a significant reduction in tooth decay rates, particularly in children.

**✓ Correct Answer: A. 1 ppm**