**Core Concept**
Iron absorption is a tightly regulated process that primarily occurs in the duodenum, the first portion of the small intestine. This process involves the reduction of non-heme iron to its ferrous state, which facilitates its absorption. The regulation of iron absorption is crucial to prevent both iron deficiency and iron overload.

**Why the Correct Answer is Right**
Iron absorption occurs chiefly from the duodenum due to the presence of specific transport proteins and enzymes. The duodenal cells express the divalent metal transporter 1 (DMT1), which reduces non-heme iron to its ferrous state and facilitates its transport across the apical membrane. The ferrous iron is then transported into the enterocyte by DMT1 and is either stored or exported into the bloodstream via the ferroportin protein. The duodenum is the primary site of iron absorption because it is rich in oxygen, which is necessary for the oxidation of non-heme iron.

**Why Each Wrong Option is Incorrect**
**Option A:** The stomach is not the primary site of iron absorption. Although gastric acid and pepsin can help break down iron-containing compounds, the stomach is not equipped with the necessary transport proteins and enzymes to facilitate iron absorption.
**Option B:** The jejunum, the second portion of the small intestine, does absorb some iron, but it is not the primary site of iron absorption. The jejunum has a lower concentration of DMT1 and ferroportin compared to the duodenum.
**Option C:** The ileum, the final portion of the small intestine, is primarily responsible for the absorption of vitamin B12 and bile salts, but it is not a significant site of iron absorption.

**Clinical Pearl / High-Yield Fact**
The regulation of iron absorption is tightly linked to the body's iron stores, with absorption increasing in response to iron deficiency and decreasing in response to iron overload. This regulatory mechanism is mediated by hepcidin, a hormone produced in the liver that controls the expression of ferroportin and thereby regulates iron export from enterocytes.

**Correct Answer:** D. The duodenum.