Oxygen molecules are small enough to fit through the phospholipid bi-layer, they do this through what process?

I think it's Simple Diffusion

Explanation:

Simple Diffusion Across the Cell (Plasma) Membrane: The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion.

i believe your answer would be c.. : )