**Core Concept**
Iron absorption is a crucial process for maintaining adequate iron levels in the body, particularly in individuals with iron deficiency anemia. The duodenum, the first part of the small intestine, plays a significant role in regulating iron absorption through specific mechanisms. The process involves the reduction of ferric iron to ferrous iron, which is then transported across the enterocytes into the bloodstream.

**Why the Correct Answer is Right**
Iron absorption occurs through an active transport mechanism in the duodenum. The **divalent metal transporter 1 (DMT1)** is a key protein responsible for the uptake of non-heme iron from the intestinal lumen into the enterocytes. The reduced ferrous iron is then bound to **transferrin** and transported across the basolateral membrane into the bloodstream. This process is essential for maintaining adequate iron levels in the body and preventing iron deficiency anemia.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because iron absorption primarily occurs in the duodenum, not the jejunum. While the jejunum is also involved in nutrient absorption, it is not the primary site for iron absorption.
**Option B:** This option is incorrect because iron absorption is an active process, not a passive one. The presence of DMT1 and other proteins in the enterocytes highlights the active nature of iron absorption.
**Option C:** This option is incorrect because iron absorption occurs primarily in the duodenum, not the colon. The colon is involved in water and electrolyte absorption, but not iron absorption.

**Clinical Pearl / High-Yield Fact**
The efficacy of iron absorption can be enhanced by the presence of **ascorbic acid** (vitamin C), which can increase the absorption of non-heme iron by up to 80%. This is why vitamin C is often recommended as a dietary supplement for individuals with iron deficiency anemia.

**Correct Answer: D. Duodenum**