If human red blood cells are placed in solutions of different osmolarity, then the 300 osmoles/L solution will be the isotonic solution.
An isotonic solution to the red blood cell would be a solution that has the same solute concentration as the red blood cell.
Isotonic solutions are different from hypotonic and hypertonic solutions. Hypotonic solutions would have lower solute concentrations than the red blood cell while hypertonic solutions would have higher concentrations than the red blood cells.
The red blood cells of humans have a certain amount of solute in their cell sap which has been approximate to be between 286 and 295 osmoles/L. Thus, the only solution close to being an isotonic solution to these cells would be the 300 osmoles/L solution.
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