Final answer:
Oxygen molecules can easily diffuse across a cell membrane because they are small and nonpolar, unlike the larger and polar glucose molecules which require transport proteins.
Step-by-step explanation:
The most accurate statement for why oxygen molecules can easily diffuse across a cell membrane while glucose molecules cannot is that oxygen molecules are very small compared to the larger glucose molecules. The cell membrane is composed of a lipid bilayer that provides a hydrophobic barrier. Small, nonpolar, or uncharged molecules like oxygen and carbon dioxide can pass through this barrier by simple diffusion, which is a form of passive transport that does not require energy. Glucose molecules, on the other hand, are too large and too polar to pass freely through the lipid bilayer and therefore require assistance from transport proteins.