Final answer:
The selective permeability of the plasma membrane allows nonpolar gases like oxygen and carbon dioxide to passively cross the membrane by simple diffusion.
Step-by-step explanation:
The characteristic of the plasma membrane that allows nonpolar gases such as oxygen and carbon dioxide to passively cross the membrane is its selective permeability.
The membrane's structure, specifically the lipid bilayer, allows small, uncharged substances like oxygen and carbon dioxide to freely diffuse across the membrane by simple diffusion.
This means that nonpolar gases can move across the membrane without the need for cellular energy.