Which statement best describes diffusion in the context of passive transport across biological systems?

• A. The movement of substances across a membrane via vesicle formation and fusion (endocytosis or exocytosis).
• B. The net movement of molecules from a region of higher concentration to a region of lower concentration due to random thermal motion, without requiring cellular energy.
• C. The movement of water across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration.
• D. The transport of molecules against their concentration gradient using ATP-driven membrane pumps.

Answer: (B)