**Core Concept**
The question is testing knowledge of the chemical composition and properties of cavity varnishes used in dentistry. Duraphat is a well-known fluoride-releasing cavity varnish. We need to understand the concentration of fluoride in this varnish to assess its efficacy in preventing caries.

**Why the Correct Answer is Right**
Duraphat cavity varnish contains sodium fluoride as its active ingredient. The fluoride concentration in Duraphat is approximately 5% w/w (5 grams of fluoride per 100 grams of varnish). This high concentration of fluoride helps to release fluoride ions slowly, providing long-lasting protection against tooth decay.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the fluoride concentration in Duraphat is not 2.5% w/w. This would be a lower concentration than what is actually present.
* **Option B:** This option is incorrect because the fluoride concentration in Duraphat is not 10% w/w. This would be a higher concentration than what is actually present.
* **Option C:** This option is incorrect because the fluoride concentration in Duraphat is not 1% w/w. This would be a lower concentration than what is actually present.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the fluoride concentration in Duraphat can vary depending on the specific product and manufacturer. However, the standard concentration of 5% w/w is widely recognized and used in dental practice.

**Correct Answer: C. 5% w/w**