Final answer:
Hydrophilic ligands cannot penetrate the hydrophobic interior of the plasma membrane, which prevents them from entering the cell. They must bind to cell-surface receptors to initiate a signal.
Step-by-step explanation:
The property that prevents the ligands of cell-surface receptors from entering the cell is their hydrophilic nature, which inhibits their ability to penetrate the hydrophobic interior of the plasma membrane. Hydrophobic or lipid-soluble ligands, such as steroid hormones, can directly diffuse through the plasma membrane. However, for hydrophilic ligands, which are water-soluble, the cell membrane acts as a barrier due to its hydrophobic core.
Therefore, the answer to the question 'What property prevents the ligands of cell-surface receptors from entering the cell?' is b. The molecules are hydrophilic and cannot penetrate the hydrophobic interior of the plasma membrane. This characteristic ensures that such ligands must bind to cell-surface receptors to initiate a signal without physically crossing the membrane.