Final answer:
The word 'bilayer' in 'phospholipid bilayer' refers to two layers of phospholipid molecules that make up cell membranes, with hydrophobic tails facing inward and hydrophilic heads facing outward.
Step-by-step explanation:
The term "bilayer" in "phospholipid bilayer" refers to a) Two layers of phospholipids. A phospholipid bilayer is the fundamental structure that composes the plasma membrane of cells. It is made up of two layers of phospholipid molecules. Each phospholipid molecule has a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails. In the bilayer, the hydrophobic tails of one layer face the tails of the other layer, forming a hydrophobic interior, while the hydrophilic heads face outward, towards the aqueous environment inside and outside the cell.