CO2 diffuses more easily through the respiratory membrane than O2 because it is:
Ans is 'b' i.e. More soluble in plasma Diffusion of gases through the tissues or respiratory membrane depends on the solubility of the gas in lipids; but since all the gases of respiratory importance are all highly soluble in lipids the diffusion through the tissues would depend on diffusion of gases in tissue water.The diffusion of gases through fluid depends on multiple factors of which the two which are characteristics of the gas itself areSolubility in fluid &Molecular weight.The diffusion coefficient of the gas is directly proportional to the solubility and inversely proportional to the square root of the molecular weight.Diffusion coefficient = Solubility/ Molecular weightDiffusion coefficient of:Oxygen--1.0 Carbon dioxide -- 20.3Carbon dioxide despite having higher molecular weight than Oxygen, has a very high diffusion coefficient because of its very high solubility in plasma (about 22 times).
Less dense
More soluble in plasma
Less molecular weight
Less PCO2 in the alveoli
Principles of Gas Exchange -  Respiratory Membrane