**Core Concept**
Haemoglobin is a crucial buffer in blood, maintaining acid-base balance. In the context of the bicarbonate buffering system, haemoglobin plays a significant role in transporting carbon dioxide (CO2) and facilitating the exchange of bicarbonate ions (HCO3-) between erythrocytes and the plasma.

**Why the Correct Answer is Right**
The correct answer is **chloride ions (Cl-)**. In the process of the bicarbonate buffering system, when carbon dioxide enters the erythrocyte, it reacts with water to form carbonic acid, which is then rapidly converted to bicarbonate and hydrogen ions. To maintain electrical neutrality, bicarbonate ions (HCO3-) diffuse out of the erythrocyte into the plasma in exchange for chloride ions (Cl-). This exchange is facilitated by the chloride-bicarbonate exchanger in the erythrocyte membrane.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the bicarbonate buffering system. The exchange of potassium ions is not directly related to the buffering of CO2.
* **Option B:** This option is incorrect because phosphate ions are not directly involved in the bicarbonate buffering system.
* **Option D:** This option is incorrect because sodium ions are not exchanged directly for bicarbonate ions in this process.

**Clinical Pearl / High-Yield Fact**
The bicarbonate buffering system is a crucial mechanism for maintaining acid-base balance in the body. It's essential to remember that the exchange of bicarbonate ions for chloride ions in erythrocytes is a critical step in this process.

**Correct Answer: C. Chloride ions.