**Core Concept**
Iron absorption is a tightly regulated process that involves the coordinated action of various proteins and hormones to ensure adequate iron levels in the body. The primary site of iron absorption is the duodenum, where iron is taken up by enterocytes via specific transporters.

**Why the Correct Answer is Right**
Iron absorption is facilitated by the divalent metal transporter 1 (DMT1) and ferroportin, which are expressed on the apical and basolateral membranes of enterocytes, respectively. The absorption of non-heme iron is enhanced by ascorbic acid, which reduces iron to its ferrous state, making it more soluble and bioavailable. Heme iron, on the other hand, is absorbed through a heme oxygenase-mediated pathway.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the mechanism of iron absorption. While ascorbic acid does enhance iron absorption, it is not the primary mechanism of iron uptake.
**Option B:** This option is incorrect because it implies that iron absorption occurs primarily in the stomach, which is not the case. The duodenum is the primary site of iron absorption.
**Option C:** This option is incorrect because it suggests that iron absorption is not regulated, which is not true. Iron absorption is tightly regulated by various proteins and hormones to prevent excessive iron accumulation.
**Option D:** This option is incorrect because it implies that iron absorption is a passive process, which is not the case. Iron absorption is an active process that requires the coordinated action of specific transporters and regulatory proteins.

**Clinical Pearl / High-Yield Fact**
The most important factor regulating iron absorption is the body's current iron status, which is sensed by the hormone hepcidin. When iron levels are high, hepcidin promotes the degradation of ferroportin, reducing iron absorption. Conversely, when iron levels are low, hepcidin levels decrease, allowing for increased iron absorption.

**Correct Answer: D.**