**Core Concept**
Fluor Protector varnish is a type of topical fluoride application used in preventive dentistry to strengthen tooth enamel and prevent dental caries. The fluoride content in these varnishes is crucial for their effectiveness.

**Why the Correct Answer is Right**
The correct answer is 5% sodium fluoride. This concentration of fluoride has been shown to be effective in preventing dental caries and strengthening tooth enamel. When applied to the teeth, the fluoride in the varnish is absorbed into the tooth enamel, making it more resistant to acid attacks from plaque bacteria and sugars in the mouth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect as the fluoride concentration may be too low to be effective.
**Option B:** This option may be incorrect as the fluoride concentration may be too high, potentially causing tooth discoloration or other side effects.
**Option C:** This option may be incorrect as the fluoride concentration may not be suitable for topical application.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the American Dental Association (ADA) has approved various fluoride varnishes, including Fluor Protector, for use in preventing dental caries. The ADA Seal of Acceptance indicates that these products have met certain standards for safety and effectiveness.

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