The word "bilayer" in the term "phospholipid bilayer" refers to which of the following?

Step 3: Consider how these molecules organize themselves in an aqueous environment. The hydrophilic heads face outward toward the water, while the hydrophobic tails face inward, away from the water. This creates a double layer, or 'bilayer,' with tails sandwiched between the heads.

Step 4: Eliminate incorrect options. Proteins and carbohydrates are not the primary components of the bilayer structure. Proteins may be embedded within or attached to the bilayer, and carbohydrates may be attached to proteins or lipids, but they do not form the bilayer itself.