**Core Concept:** Neutral sodium fluoride is a type of fluoride compound used for dental caries prevention. It acts as a caries inhibitor by promoting remineralization of tooth enamel and reducing demineralization. Sodium fluoride is commonly used in toothpaste, mouth rinses and water fluoridation programs. It binds to hydroxyapatite crystals in tooth enamel, making it more resistant to acid attacks by plaque bacteria.

**Why the Correct Answer is Right:** 2% neutral sodium fluoride refers to the concentration of fluoride ions in the compound. In this case, 2% means that there are 2 parts fluoride ions per 100 parts of the solution. Among the given options, only option D (2% sodium fluoride) contains the correct concentration of fluoride ions. The other options represent different concentrations and are thus incorrect.

**Why Each Wrong Option is Incorrect:**

A. 0.2% sodium fluoride - This concentration is much lower than the recommended optimal dose for caries prevention and may not provide sufficient fluoride exposure to teeth.
B. 10% sodium fluoride - This is an excessively high concentration of sodium fluoride that may lead to fluorosis, a dental disorder characterized by enamel discoloration and other fluoride-related effects.
C. 50% sodium fluoride - This concentration is extremely high and not suitable for oral use, as it can cause severe systemic toxicity, including fluoride poisoning, if ingested.

**Clinical Pearl:** Sodium fluoride at optimal concentrations (1-2%) is a safe and effective method of preventing dental caries. It is essential to understand the correct fluoride concentrations to prevent adverse effects associated with excessive fluoride exposure.

**Correct Answer:** D. 2% sodium fluoride