An isotonic solution contains equal solute concentration on both sides.

Rationale: In an isotonic solution, solute concentration is equal on both sides of a cell membrane, maintaining cell equilibrium. This prevents water from moving in or out of the cell, ensuring stability. This balance prevents cell shrinkage or swelling. This is a fundamental concept in biology and crucial for cellular processes. Other choices are incorrect as they do not accurately describe the characteristics of an isotonic solution.