The cell membrane separates the intracellular environment from the extracellular environment outside of the cell. A phospholipid is composed of a phosphate group and a lipid. It is composed of a hydrophilic, polar phosphate head and two hydrophobic fatty acid tails.
The structure of the bilayer is such that the tails are shielded from contact with water, creating a selectively-permeable barrier which will be impermeable to most hydrophilic molecules.
So, the answer is A) To maintain balance of water and nutrients in the cell.