The correct answer is option (3) as nearly 20−25 precent of CO2 is transported by RBCs whereas 70 percent of it is carried as bicarbonate. About 7 percent of CO2 is carried in a dissolved state through plasma. The binding of CO2 with haemoglobin is related to the partial pressure of CO2.pO2 is a major factor which could affect this binding. When pCO2 is high and pO2 is low as in the tissues, more binding of CO2 occurs.Percentage of a gas transported depends upon its partial pressure. If partial pressure decreases, the transportation of gases also decrease with haemoglobin. So, Reason is the correct explanation of Assertion.