**Core Concept**
The absorption of bicarbonate ions (HCO3-) is a critical process in maintaining acid-base balance. HCO3- is a key buffer system in the body, and its absorption in the intestines is essential for preventing metabolic acidosis. The absorption of HCO3- in the intestines is facilitated by the enzyme carbonic anhydrase.

**Why the Correct Answer is Right**
The correct answer is **duodenum**. The duodenum is the first part of the small intestine and is the primary site of HCO3- absorption. Carbonic anhydrase, an enzyme that catalyzes the reversible reaction between carbon dioxide (CO2) and water (H2O) to form HCO3- and hydrogen ions (H+), is present in the brush border membrane of the duodenal epithelial cells. As HCO3- is absorbed, it helps to neutralize excess H+ ions in the blood, maintaining acid-base balance. The absorption of HCO3- in the duodenum is also facilitated by the presence of sodium ions (Na+), which helps to drive the absorption of HCO3- through the sodium-hydrogen exchanger (NHE) mechanism.

**Why Each Wrong Option is Incorrect**
**Option A:** jejunum - While the jejunum is also a part of the small intestine, it is not the primary site of HCO3- absorption. The jejunum is more involved in the absorption of carbohydrates, proteins, and fats.
**Option B:** ileum - The ileum is the final part of the small intestine, and while it does absorb some HCO3-, it is not the primary site of HCO3- absorption.
**Option C:** colon - The colon is primarily involved in water and electrolyte absorption, and while it does absorb some HCO3-, it is not the primary site of HCO3- absorption.

**Clinical Pearl / High-Yield Fact**
The absorption of HCO3- in the duodenum is influenced by the presence of sodium ions and the activity of carbonic anhydrase. This process is crucial for maintaining acid-base balance and preventing metabolic acidosis.

**Correct Answer: D. Duodenum**