When molecular oxygen moves through a membrane, which of the following types of transport is used?

The answer is a) simple diffusion.

Explanation: Molecular oxygen (O₂) is a small, nonpolar molecule that can easily pass through the lipid bilayer of cell membranes. This movement occurs without any assistance from proteins or energy. In simple diffusion, substances move down their concentration gradient—meaning from an area of higher concentration to an area of lower concentration—until equilibrium is reached. Since oxygen can diffuse directly through the membrane, it does not require facilitated diffusion, primary active transport, or secondary active transport.