In this qn, we need to make use of the graph given. For %change in length above 0, the amount of water molecules in sugar solution is higher than cell sap. As water molecules enter cell sap via osmosis and increase length. Same can be said for % below 0, the amount of water molecules in cell sap is higher than sugar solution so it reduces in length as water molecules exit. Thus for both to be at the same water potential, there is no change in length. ( at 0)