Final answer:
A 0.9% NaCl solution is considered isotonic because its concentration of solute is equal to the concentration of solute inside the cells.
Step-by-step explanation:
An isotonic solution is one in which the concentration of solute is equal to the concentration of solute inside the cells. In the case of a 0.9% NaCl solution, it is considered isotonic because the concentration of NaCl in the solution is the same as the concentration of NaCl in the cells. This means that there will be no net movement of water into or out of the cells.