The right answer is b. cell membrane.
The cell membrane is a bilayer of phospholipids between which proteins are found (facilitate the movement of materials).
Phospholipids contain a polar head, which attracts water. The non-polar tail is hydrophobic and avoids water. The water surrounds the cell and is inside.
The plasma membrane controls homeostasis: The internal environment of the body (the cell) remains unchanged, even if the external environment varies.