**Core Concept**
Iron absorption is a crucial process in maintaining adequate iron levels in the body, particularly in the setting of iron deficiency anemia. The active absorption of iron occurs primarily in the duodenum and upper jejunum, where specialized transport mechanisms facilitate the uptake of non-heme iron.

**Why the Correct Answer is Right**
Iron absorption is mediated by the divalent metal transporter 1 (DMT1), a protein located on the apical membrane of duodenal enterocytes. DMT1 facilitates the uptake of non-heme iron, which is then transported into the enterocyte via the ferrous iron transporter (FRT). Once inside the enterocyte, iron is either stored in ferritin or exported into the bloodstream via the ferroportin transporter. The acidic environment of the duodenum, created by the secretion of hydrochloric acid, also enhances iron absorption by converting non-heme iron into a more soluble and bioavailable form.

**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because iron absorption primarily occurs in the duodenum and upper jejunum, not the ileum.
* **Option B:** This is incorrect because while the stomach does play a role in iron absorption by secreting hydrochloric acid, it is not the primary site of active iron absorption.
* **Option D:** This is incorrect because the colon is not a major site of iron absorption; rather, it is primarily involved in the absorption of water and electrolytes.

**Clinical Pearl / High-Yield Fact**
The acidic environment of the duodenum is crucial for iron absorption, and conditions that impair gastric acid secretion, such as atrophic gastritis or proton pump inhibitor therapy, can lead to reduced iron absorption and an increased risk of iron deficiency anemia.

**Correct Answer: C.