**Core Concept:**
The respiratory membrane is a selective barrier that allows certain gases to pass through more easily than others due to their size, polarity, and diffusibility. Oxygen (O2) and carbon dioxide (CO2) are the main gases involved in gas exchange in the lungs.

**Why the Correct Answer is Right:**
CO2 diffuses more easily through the respiratory membrane than O2 due to several factors. Firstly, CO2 is a small, nonpolar molecule compared to O2, which is a larger, polar molecule. Smaller and nonpolar molecules have less resistance to diffusion across biological membranes, including the respiratory membrane. Secondly, CO2 has a higher solubility in blood plasma compared to O2, which further facilitates its transport within the body.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because the size and polarity of CO2 and O2 molecules, as explained above, favor CO2 diffusion.
B. Although CO2's solubility in blood plasma may be higher than O2's, this is not the primary reason for CO2's easier diffusion. The molecular size and polarity play a more significant role.
C. This option is incorrect as the respiratory membrane is selective and allows both O2 and CO2 to pass through, but CO2 diffuses more easily due to its molecular properties.
D. This option does not address the molecular properties that make CO2 easier to diffuse through the respiratory membrane.

**Clinical Pearl:**
Understanding the molecular properties of gases is crucial for understanding their diffusion across biological membranes. This principle applies not only to the respiratory system but also to other physiological processes involving gas diffusion, such as blood-brain barrier permeability and nutrient absorption in the gut.

**Correct Answer:**
C. The correct answer is C, which highlights the selectivity of the respiratory membrane and the influence of CO2 and O2 molecule properties on their diffusion. The membrane allows both gases to pass, but CO2 diffuses more easily due to its smaller size and nonpolar nature.