Final answer:
Protein is the substance least able to diffuse through the lipid bilayer of the plasma membrane. Small, uncharged substances like oxygen and carbon dioxide can pass freely, whereas proteins cannot due to their size and polarity.
Step-by-step explanation:
The substance least able to diffuse through the lipid bilayer of the plasma membrane is C. protein. Small, uncharged substances such as oxygen and carbon dioxide, as well as hydrophobic molecules, can pass through by simple diffusion, exploiting their small size and lack of charge. Proteins, however, are typically large, complex molecules and are often charged or polar, which prevents them from easily passing through the hydrophobic interior of the lipid bilayer.
Lipid-soluble materials and nonpolar molecules like oxygen and carbon dioxide have no charge, allowing them to move across plasma membranes by simple diffusion. In contrast, polar substances and ions, such as sodium, potassium, calcium, and chloride, cannot pass through the lipid bilayer without assistance because their charge prevents them from doing so. This assistance may come through various transmembrane proteins that facilitate the transport of these substances.
Water, despite being a polar molecule, is small enough to pass through the plasma membrane more easily than proteins, although not as easily as oxygen or carbon dioxide. For proteins, special mechanisms such as protein channels and transporters are required to facilitate their movement across the membrane.