A solution to be isotonic refers to b) The concentration of solute in the environment and the concentration inside the cell are at equilibrium.
Isotonic solution is the solution that have equal concentration of solute on both sides of the solution separated by a membrane.
If a cell is kept in an isotonic solution that means the concentration of the solute inside the cell is the same as that of the outside then the movement of solvent does not occur between the cell and the external environment.
The osmotic pressure inside the cell and outside the cell across the semipermeable membrane of the cell becomes the same due to equal concentration of the solute on each side.
0.9% normal saline is an example of isotonic solution.
