Question

diffusion of sodium (na+) across the membrane requires help from a protein channel because it is ______, unlike oxygen, which can pass unassisted through the membrane.
charged
small
nonpolar

Explanation:

Cell membranes are composed of a lipid - bilayer which is hydrophobic in the interior. Charged ions like sodium ($Na^+$) cannot easily cross this hydrophobic region and require protein channels for diffusion. Oxygen is small and non - polar, allowing it to pass unassisted.

Answer:

charged