Final answer:
The movement of particles across the plasma membrane is most effectively promoted when the substance is small, with lipid-soluble materials passing through more readily than hydrophilic substances.
Step-by-step explanation:
The movement of particles across the plasma membrane is promoted when the substance is small (B).
Plasma membranes are amphipathic, possessing both hydrophilic and hydrophobic regions. This unique structure allows certain materials to pass through more easily than others. Lipid-soluble materials and those with a low molecular weight can slip through the hydrophobic core of the membrane efficiently. For example, fat-soluble vitamins and nonpolar molecules like oxygen and carbon dioxide diffuse through via simple diffusion. Conversely, hydrophilic substances and those that are lipid insoluble face challenges in crossing the membrane unaided, often requiring specialized transport mechanisms such as protein channels or carrier proteins for processes like facilitated diffusion.