Diffuion describes the movement of particles through random motion from areas of higher concentration to regions of lower concentration, while
osmosis describes the movement of water across a selectively permiable membrane driven by a difference in solute concentrations on the two sides of teh membrane.
The red blood cell is surrounded by a membrane enclosing an area with a higher concentration of solutes inside than the surrounding water. The distilled water will move across the membrane from the area of lower solute concentration (outside the cell) to the area of higher concentration (inside the cell).
Thus, the cell will gain water through osmosis.