Which sentence best describes a cell that is isotonic for a substance?

the concentration of the solute is lower inside the cell membrane.
the solute is absent from both sides of the cell membrane.
the solute concentration is equal on both sides of the cell membrane.
the concentration of the solute is higher inside the cell membrane.

The answer would be C) The solute concentration is equal on both sides of the cell membrane.
This is correct because in an isotonic solution, they would have the exact salt concentration as the blood surrounding the cell. There won't be an effect on the surrounding cells. So, the cells won't gain or loose any water. 
Hope this helps!:)

Um, you are going to have to be more clear on the question. can you update the question and make it more clear?