Why must the red blood cells and the solution in the beaker be isotonic?

If a cell is placed in an isotonic solution, there will be no net flow of water into or out of the cell, and the cell's volume will remain stable. If the solute concentration outside the cell is the same as inside the cell, and the solutes cannot cross the membrane, then that solution is isotonic to the cell.

i think it's c. if you look search electronic symbol on google and then click on wiki it shows you a