**Core Concept**
The acidulated phosphate fluoride (APF) solution is a widely used topical fluoride agent in dentistry. It contains sodium monofluorophosphate (Na2PO3F) as the source of fluoride ions, which are released when the solution comes into contact with the tooth surface.

**Why the Correct Answer is Right**
When the APF solution is applied to the tooth, the sodium monofluorophosphate reacts with water to release fluoride ions (F-) and sodium dihydrogen phosphate. This reaction is catalyzed by the acidic pH of the solution, which helps to accelerate the release of fluoride ions. The resulting fluoride ion concentration in the solution is typically around 1.23% (12,300 ppm).

**Why Each Wrong Option is Incorrect**
* **Option A:** There is no information available to suggest that the fluoride ion concentration in APF solution is around 0.5% (5,000 ppm).
* **Option B:** The fluoride ion concentration in APF solution is not around 2.5% (25,000 ppm), as this would be an excessively high concentration for topical application.
* **Option D:** There is no evidence to suggest that the fluoride ion concentration in APF solution is around 0.05% (500 ppm), as this would be too low for effective caries prevention.

**Clinical Pearl / High-Yield Fact**
The acidulated phosphate fluoride solution is a popular choice for topical fluoride application due to its ease of use and effectiveness in preventing dental caries. It is essential to note that the fluoride ion concentration in APF solution is critical in determining its efficacy, and concentrations outside the recommended range may lead to reduced effectiveness or increased risk of toxicity.

**Correct Answer:** C. 1.23% (12,300 ppm)