**Core Concept**
Carbon dioxide (CO₂) is transported in the blood primarily as bicarbonate ions (HCO₃⁻) formed via the carbonic acid-bicarbonate buffer system. This process occurs in red blood cells through enzymatic conversion of CO₂ and water into carbonic acid, which then dissociates into H⁺ and HCO₃⁻, with most of the HCO₃⁻ being transported out of red blood cells into plasma.

**Why the Correct Answer is Right**
Approximately 70% of CO₂ is transported as bicarbonate (HCO₃⁻) in plasma. In red blood cells, CO₂ reacts with water to form carbonic acid (H₂CO₃) via carbonic anhydrase, which rapidly dissociates into H⁺ and HCO₃⁻. The HCO₃⁻ diffuses into the plasma, while H⁺ is buffered by hemoglobin. This mechanism efficiently handles the large volume of CO₂ produced by metabolism and is the main pathway for CO₂ elimination.

**Why Each Wrong Option is Incorrect**
Option A: Dissolved CO₂ accounts for only about 5% of total CO₂ transport and is negligible compared to bicarbonate.
Option B: Carbonic acid is a transient intermediate; it does not represent the primary form of CO₂ transport in blood.
Option C: Carbamino-hemoglobin (CO₂ bound to hemoglobin) accounts for about 20–25% of CO₂ transport, less than bicarbonate.

**Clinical Pearl / High-Yield Fact**
In respiratory acidosis, the body compensates by increasing bicarbonate levels, reflecting the central role of the bicarbonate buffer system in CO₂ homeostasis. Remember: **"70% bicarbonate, 20% carbamino, 5% dissolved"** — this ratio is essential for understanding CO₂ transport.

✓ Correct Answer: D. Bicarbonate