## Core Concept
The question pertains to the concentration of a sodium fluoride (NaF) solution, specifically 0.02% NaF, and its equivalent expression in parts per million (ppm) or milligrams per liter (mg/L).

## Why the Correct Answer is Right
0.02% NaF is equivalent to 200 ppm or 200 mg/L. This is because 0.02% means 0.02 grams per 100 milliliters (or grams) of solution. To convert this to ppm, we use the conversion factor where 1% = 10,000 ppm. Therefore, 0.02% = 0.02 * 10,000 ppm = 200 ppm. This matches option .

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately represent 0.02% NaF in ppm or mg/L.
- **Option B:** This option is incorrect for the same reason as option A; it does not correctly convert 0.02% NaF to ppm or mg/L.
- **Option D:** This option is incorrect as it also does not accurately represent the correct conversion of 0.02% NaF.

## Clinical Pearl / High-Yield Fact
A key point to remember is that fluoride solutions, particularly those used in dentistry for the prevention of dental caries, are often expressed in terms of ppm (parts per million) of fluoride ions. The concentration of 0.02% NaF, which is commonly used in professional fluoride applications, equates to 200 ppm of fluoride. This concentration is significant for its effectiveness in preventing dental caries while minimizing the risk of dental fluorosis.

**Correct Answer:** . 200 ppm.