**Core Concept**
Fluoride absorption occurs primarily in the gastrointestinal tract, with the majority being absorbed in the stomach and small intestine. The amount of fluoride absorbed depends on various factors, including the dose, pH, and presence of other substances.

**Why the Correct Answer is Right**
The maximum amount of fluoride is absorbed when it is in the form of sodium fluoride (NaF) and is administered orally. This is because NaF is more soluble and easily absorbed in the stomach and small intestine. The acidic environment of the stomach also facilitates the dissociation of NaF into sodium ions and fluoride ions, which are then absorbed by the body. Additionally, the presence of other substances like food can increase the absorption of fluoride by forming complexes that are more easily absorbed.

**Why Each Wrong Option is Incorrect**
**Option A:**
Fluoride can be absorbed through the skin and mucous membranes, but this is not the primary route of absorption, and the amount absorbed is much smaller compared to oral administration.

**Option B:**
Calcium fluoride (CaF2) is less soluble than sodium fluoride and is not as easily absorbed by the body.

**Option C:**
Stomach acid (HCl) can help dissolve and facilitate the absorption of fluoride, but it is not the most effective form of fluoride absorption.

**Clinical Pearl / High-Yield Fact**
The optimal level of fluoride in drinking water is between 0.7 and 1.2 milligrams per liter (mg/L), as this range has been shown to reduce dental caries while minimizing the risk of dental fluorosis.

**Correct Answer: B. Calcium fluoride (CaF2)**