Protein. i learned in in 8th grade.
The correct answer is small hydrophobic molecules.
The smaller the molecule and the more hydrophobic, or nonpolar, it is, the more rapidly it will diffuse across a membrane. It is because of the composition of membrane (lipid bilayer). By contrast, membrane is highly impermeable to charged molecules (ions), no matter how small they are.
I believe the answer is : D
