**Core Concept**
Fluoride defluoridation is a process that reduces the concentration of fluoride in water or other substances, often necessary in areas with high levels of naturally occurring fluoride. This concept is crucial in dentistry, especially in communities prone to dental fluorosis due to excessive fluoride intake.

**Why the Correct Answer is Right**
Alumina, or aluminum oxide, is used as a defluoridating agent. It works by reacting with fluoride ions to form insoluble aluminum fluoride, which can then be removed from the water or substance. This process is often employed in water treatment plants to reduce the fluoride content to safe levels for consumption. The effectiveness of alumina in defluoridation lies in its ability to selectively bind to fluoride ions, making it an ideal agent for this purpose.

**Why Each Wrong Option is Incorrect**
**Option A:** Duraphat is a fluoride-releasing toothpaste, used to prevent dental caries, not for defluoridation.

**Option B:** Fluorprotector is a type of fluoride varnish used in dental practice, also for preventing dental caries, not for removing fluoride.

**Option C:** Brudevold's solution is a fluoride solution used in dental research, not for defluoridation.

**Clinical Pearl / High-Yield Fact**
When treating patients with high levels of fluoride, it's essential to consider the source of the fluoride (e.g., water, toothpaste) and take steps to reduce exposure. This can involve using defluoridating agents like alumina or switching to fluoride-free products.

**✓ Correct Answer: D. alumina**