Why can only water molecules enter the cell during osmosis?

Water molecules can enter the cell during osmosis primarily because they are smaller than protein molecules and other larger macromolecules. Osmosis specifically refers to the movement of water across a semi-permeable membrane, and this process relies on the concentration gradient of water. The ability of water molecules to diffuse freely through the cell membrane is facilitated by their small size, which allows them to pass through the lipid bilayer of the membrane or through specialized channels called aquaporins.

Other options present concepts that might not directly relate to why only water molecules can enter the cell during osmosis. For instance, the size and polarity of molecules play a crucial role in their ability to penetrate cell membranes, and water's small size is a key factor in its movement during osmosis.