Living matter is at least sixty percent water. it makes sense, then, that cell membranes are made of phospholipids because
a) phosphates dissolve in water.
b) the bilayer acts to repel water.
c) the lipids form an insoluble barrier.
d) the lipids mix easily in a water environment.

The answer is C) the lipids form an insoluble barrier.

Phospholipids, a type of lipids, don’t dissolve in water. Membrane-forming lipids have like other lipids hydrophobic region, but also in addition, they have hydrophilic region.  In the cell membrane, they are organized in a lipid bilayer. In each layer, hydrophilic heads face a solution, and hydrophobic tails are inside the layer. That way, phospholipids are in contact with water, but still present a barrier because of their hydrophobic tails inside the layer.