Isotonic solutions have:[A] Same boiling point[B] Same vapour pressure[C] Same melting point[D] Same osmotic pressure.

To determine which characteristic applies to isotonic solutions, let’s first clarify what an isotonic solution is. Isotonic solutions are solutions that have the same osmotic pressure.

Explanation of Options:
[A] Same boiling point: Isotonic solutions do not necessarily have the same boiling point. The boiling point of a solution depends on its solute concentration and the nature of the solute itself.

[B] Same vapor pressure: Isotonic solutions will have similar vapor pressures, but this is not a definitive characteristic of isotonic solutions. The vapor pressure of a solution changes based on the solute concentration and type.

[C] Same melting point: The melting point of solutions varies with the concentration and type of solute present. Therefore, isotonic solutions do not have the same melting point.

[D] Same osmotic pressure: This is the defining characteristic of isotonic solutions. Isotonic solutions have the same osmotic pressure, which means that when separated by a semipermeable membrane, there will be no net movement of water across the membrane.

Conclusion:
The correct answer is [D] Same osmotic pressure. Isotonic solutions are defined by having equal osmotic pressures, which means that they will not cause a net movement of water when separated by a semipermeable membrane.