Let's analyze the given statements:

Assertion A: During the transportation of gases, about 20-25 percent of CO$_2$ is carried by Haemoglobin as carbamino-haemoglobin.

Reason R: This binding is related to high pCO$_2$ and low pO$_2$ in tissues.

Explanation:

1. Assertion A: It is true that during the transportation of gases in the blood, around 20-25% of carbon dioxide (CO$_2$) is indeed carried by hemoglobin in the form of carbamino-haemoglobin. Hemoglobin not only carries oxygen (O$_2$) but also plays a role in transporting CO$_2$ from the tissues back to the lungs.

2. Reason R: This is also true. The binding of CO$_2$ to hemoglobin to form carbamino-haemoglobin is influenced by the partial pressures of CO$_2$ (pCO$_2$) and O$_2$ (pO$_2$). In tissues where cellular respiration is occurring, the pCO$_2$ is high, and the pO$_2$ is low. This environment favors the formation of carbamino-haemoglobin, as hemoglobin has a higher affinity for CO$_2$ under these conditions.

Therefore, both Assertion A and Reason R are true. Additionally, Reason R correctly explains why Assertion A is true, as the conditions in the tissues (high pCO$_2$ and low pO$_2$) directly promote the formation of carbamino-haemoglobin.

Correct Answer: Option C - Both A and R are true and R is the correct explanation of A.