Final answer:
Sodium ions require a specific transport mechanism to cross cell membranes because their charge prevents passage through the hydrophobic interior.
Step-by-step explanation:
The substance that doesn't cross membranes unless a specific transport mechanism is present is d) Sodium ions. Water (a), oxygen (b), and glucose (c) can cross cell membranes more readily. For example, oxygen can pass through by simple diffusion due to its small size and nonpolar nature, and water can move through membranes using aquaporins. However, sodium ions are charged and cannot diffuse through the hydrophobic interior of the membranes. Thus, they require specific ion channels or transporters to move across.