Final answer:
Osmosis is the diffusion of water through a selectively permeable membrane from an area of higher water concentration to lower water concentration. The correct answer to the student's question is C. Water, as osmosis pertains specifically to the movement of water molecules.
Step-by-step explanation:
Osmosis Definition
Osmosis is the diffusion of water across a selectively permeable membrane. It is a biologically important process facilitating the transport of water molecules depending on the solute concentration gradient. In simple terms, water tends to move from an area where it is more concentrated (meaning fewer solutes) to an area where it is less concentrated (where there are more solutes).
During osmosis, a semipermeable membrane allows the passage of water but not solutes. Therefore, water moves to try to equalize solute concentrations on both sides of the membrane. This is critical for maintaining proper cellular function and integrity and occurs naturally in various biological contexts, such as within kidney tubules or red blood cells.
When considering the student's multiple-choice question, the correct answer is C. Water, because osmosis specifically refers to the movement of water, and not solutes like urea, oxygen, or sodium, across a membrane.