"A" is the correct option.
Functions of the cell membrane include:
1. It maintains the physical integrity of the cell by enclosing the cytoplasm and organelles within it
2. Forms a barrier between the inside of the cell and the environment outside the cell enabling different chemical groups to exist on each side of the cell.
3. Active transport. The cell membrane allows active transport of specific molecules across the cell membrane in either direction, that is into and out of the cell.
