What does osmosis specifically refer to?

Osmosis specifically refers to the transfer of water through a selectively permeable membrane. In biological systems, this process occurs when water molecules move from an area of lower solute concentration to an area of higher solute concentration. The goal of osmosis is to achieve equilibrium in solute concentrations on both sides of the membrane, but it fundamentally involves the movement of water rather than solutes or proteins.

The other choices involve different processes: the movement of solutes through a membrane pertains to diffusion or active transport mechanisms, the equalization of gas concentration relates to gas exchange processes, and the diffusion of proteins typically requires specific mechanisms that differ from osmotic processes.