**Core Concept**
Bicarbonate absorption is a critical process in maintaining acid-base balance in the body, primarily occurring in the kidneys and gastrointestinal tract. The **proximal convoluted tubule (PCT)** of the nephron is key in renal bicarbonate reabsorption.

**Why the Correct Answer is Right**
The correct answer is related to the primary site of bicarbonate reabsorption. In the **PCT**, nearly 80% of filtered bicarbonate is reabsorbed back into the bloodstream. This process involves the conversion of bicarbonate to carbon dioxide and water within the tubular lumen, facilitated by **carbonic anhydrase**, and then the diffusion of carbon dioxide into the tubular cells where it's converted back to bicarbonate.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, while some bicarbonate absorption does occur here, it's not the primary site.
**Option B:** Incorrect as this segment primarily deals with water and electrolyte balance rather than bicarbonate absorption.
**Option C:** Incorrect because, although bicarbonate can be absorbed in the intestines, the question context suggests renal physiology.

**Clinical Pearl / High-Yield Fact**
A crucial point to remember is that **carbonic anhydrase inhibitors**, like acetazolamide, can decrease bicarbonate reabsorption, leading to metabolic acidosis.

**Correct Answer:** D. Proximal convoluted tubule