**Core Concept**
The transport of carbon dioxide (CO2) in the blood is a critical physiological process that involves the coordination of multiple mechanisms to maintain acid-base balance. CO2 is produced as a byproduct of cellular metabolism and must be efficiently transported from the tissues to the lungs for exhalation.

**Why the Correct Answer is Right**
The majority of CO2 in the blood is transported as bicarbonate ions (HCO3-). This process occurs in two steps: first, CO2 diffuses into the red blood cells (RBCs) and reacts with water to form carbonic acid (H2CO3), which then dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). This reaction is catalyzed by the enzyme carbonic anhydrase. The bicarbonate ions then bind to the hemoglobin molecule, allowing for efficient transport of CO2 to the lungs. This mechanism is essential for maintaining acid-base balance in the body.

**Why Each Wrong Option is Incorrect**
**Option A:** It is mostly transported as carboxyhemoglobin. This is incorrect because carboxyhemoglobin is formed when CO binds to hemoglobin, not CO2. Carboxyhemoglobin is a toxic compound that can cause tissue hypoxia.
**Option B:** As dissolved CO2. This is incorrect because while some CO2 does dissolve in plasma, the majority is transported as bicarbonate ions.
**Option D:** Due to CO2 molecules attached to hemoglobin. This is incorrect because while CO2 does bind to hemoglobin, it is not the primary mechanism of transport. The correct mechanism involves the formation of bicarbonate ions.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the transport of CO2 in the blood is tightly regulated by the enzyme carbonic anhydrase, which catalyzes the reaction of CO2 with water to form bicarbonate ions. Inhibitors of carbonic anhydrase, such as acetazolamide, can impair CO2 transport and lead to acid-base disturbances.

✓ Correct Answer: C. As bicarbonates