Isotonic solutions are the solutions having the same:

Options

A. surface tension
B. vapour pressure
C. osmotic pressure
D. viscosity

Answer

To solve the question regarding isotonic solutions, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Definition of Isotonic Solutions**:
Isotonic solutions are defined as solutions that have the same osmotic pressure. This means that when two solutions are separated by a semipermeable membrane, there will be no net movement of solvent molecules between them.

2. **Identify the Key Concept**:
The key concept here is osmotic pressure. Osmotic pressure is the pressure required to prevent the flow of solvent into a solution via osmosis. For two solutions to be isotonic, their osmotic pressures must be equal.

3. **Examine the Options**:
- **Surface Tension**: This refers to the cohesive forces at the surface of a liquid. It is not related to isotonicity.
- **Vapor Pressure**: This is the pressure exerted by a vapor in equilibrium with its liquid or solid phase. It does not determine isotonic solutions.
- **Osmotic Pressure**: This is the correct answer as it defines the condition for isotonic solutions.
- **Viscosity**: This refers to the thickness or stickiness of a fluid, which is unrelated to the concept of isotonic solutions.

4. **Conclusion**:
Since isotonic solutions have the same osmotic pressure, the answer to the question is that isotonic solutions are those that have the same **osmotic pressure**.

### Final Answer:
Isotonic solutions are the solutions having the same **osmotic pressure**.

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