The correct answer is B.
The cell membrane is a two-layer lipid membrane that surrounds and protects the cell. It allows only certain molecules to enter and leave the cell, that's why we call it semi-permeable. The membrane is permeable only for small,uncharged molecules which enter and leave in the process of osmosis. All other types of molecules require a special transport protein or a channel in order to be transported trough the cell membrane.
