Since no options are given, Cell membranes protect the cells that is why they are called the gatekeepers of the cell. A cell membrane can be internal or external like that of the case of eukaryotic cells with internal cell membrane. It is mostly made up of glycerophospholipids. This compound like other lipids is also soluble in water however, their unique geometry allows them to aggregate into bilayers without too much energy input. Cholesterol can also be found in the cell membranes however, it is not as abundant as the glycerophospholipids.
Which is true of the phospholipid bilayer of the cell membrane