Answer :
Well, probably a non-polar fat soluble molecule, because Ions and Large Polar Molecules cannot.
Water diffusion is called osmosis. Oxygen is a small molecule and it's non polar, so it easily passes through a cell membrane. Carbon dioxide, the byproduct of cell respiration, is small enough to readily diffuse out of a cell. Small uncharged lipid molecules can pass through the lipid innards of the membrane.