Diffusion is the random movement of particles from regions of higher concentration to lower concentration, and it doesn't require energy from the cell. Osmosis is a specific kind of diffusion: water moving across a selectively permeable membrane toward the side with higher solute concentration (lower water potential). The statement that ties these ideas together is the best because it defines diffusion in general and then identifies osmosis as diffusion of water through a membrane, making the relationship clear. Osmosis is water-specific, and it depends on the membrane being permeable to water but not necessarily to solutes, while diffusion applies to any substance moving down its own concentration gradient and can occur in gases, liquids, or solids. The other options misstate these points: diffusion is not energy-requiring, and it is not limited to liquids.