The correct answer is Phospholipids.

- The cell membrane is primarily composed of a bilayer structure formed by phospholipids.
- Each phospholipid molecule has a hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails.
- This unique amphipathic nature causes the phospholipids to arrange themselves into a bilayer with the hydrophobic tails facing inward, away from water, and the hydrophilic heads facing outward, toward the aqueous environments inside and outside the cell.
- This arrangement provides the membrane with a flexible yet stable barrier that controls the passage of substances in and out of the cell.

- While cholesterol is also present in the membrane and helps to modulate its fluidity and stability, it does not form the bilayer structure itself. Similarly, glycoproteins and other proteins serve functional roles in cell recognition and transport but are not the primary structural components of the bilayer.
- Triglycerides are storage lipids and are not involved in forming the membrane structure.

Key points:
- Phospholipids form the basic bilayer structure of the cell membrane.
- The bilayer arrangement is due to the amphipathic nature of phospholipids.
- Cholesterol modulates membrane fluidity but does not form the bilayer.
- Proteins and glycoproteins have functional roles, not structural bilayer roles.
- Triglycerides are storage molecules, not membrane components.

Reference: “Molecular Biology of the Cell,” 6th Edition, Alberts et al., Chapter 10, pp. 250-255.