C. Water molecules flow in both directions at the same rate.
Explanation:
The relative concentration of solutions indicates the direction of movement of water across the semi-permeable membrane. An isotonic solution is that which has the same osmolarity of a cell. As a result, there is no concentration gradient for water across the cell membrane. In this case, water molecules will move through the plasma membrane in both directions with the same rate. Thus, there will be no net movement of water into or out of the cell.
