Final answer:
Transporter proteins, or integral proteins, are the parts of the cell membrane responsible for facilitating the movement of small hydrophilic molecules, as they cannot pass through the hydrophobic lipid bilayer unaided.
Step-by-step explanation:
The correct answer to the question of which part of the membrane could be involved in the movement of a small hydrophilic molecule is d) Transporter. Transporters, which are also known as integral proteins, form channels and carriers in the cell membrane that facilitate the movement of hydrophilic molecules across the phospholipid bilayer. Phospholipids, which form the bilayer, and cholesterol, which provides flexibility to the membrane, are not typically involved in the active transport of hydrophilic substances, since they are both hydrophobic in nature. However, it is the transporter proteins that provide the means for such molecules to cross the hydrophobic core of the membrane, as small hydrophilic molecules cannot easily pass through the nonpolar lipid bilayer without assistance.