What kind of membrane is being used?

Both osmosis and diffusion equalize the concentration of two solutions and occur with no additional energy. However, the difference is that osmosis is process in which water (or another solvent) moves from one solution to another. Think of how a plant root swells when it takes in water. Osmosis cannot occur if there is not at least a semipermeable membrane present.

In diffusion, large concentrations of a solvent are diffused in an area, like how perfume spreads out in the air after being sprayed. Since it is the "water level" that resulted from the process in this question, the answer is osmosis because water moves in the osmosis. The membrane type in this example must be semipermeable.