Final answer:
Ions are the molecules that would be blocked by a cell membrane because their charge prevents them from passing through the hydrophobic interior of the phospholipid bilayer without the help of specific transport mechanisms.
Step-by-step explanation:
The molecule that would be blocked by a cell membrane is B. Ions.
The cell membrane consists of a lipid bilayer which allows small, non-charged molecules like simple alcohols, water, and dissolved oxygen to pass through by simple diffusion. However, due to their charge, ions cannot diffuse through the hydrophobic interior of the phospholipid bilayer. Specific transport mechanisms such as ion channels or transporters are required for ions like sodium, potassium, calcium, and chloride to move across the plasma membrane.