Assertion : Sound waves travelling in air cannot be polarised.
Reason : Polarisation is property of light waves only.

Options

A. If both assertion and reason are correct and reason is a correct explanation of the assertion.
B. If both assertion and reason and correct but reason is not the correct explanation of assertion.
C. If assertion is correct but reason is incorrect.
D. If assertion is incorrect but reason is correct.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Sound waves travelling in air cannot be polarised."
- This is true because sound waves are longitudinal waves. In longitudinal waves, the particles of the medium vibrate parallel to the direction of wave propagation. Thus, they cannot exhibit polarization.

2. **Understanding the Reason**:
- The reason states that "Polarisation is a property of light waves only."
- This statement is incorrect. While it is true that light waves (which are transverse waves) can be polarized, other types of waves, such as certain mechanical waves, can also exhibit polarization if they are transverse in nature.

3. **Conclusion**:
- Since the assertion is true (sound waves cannot be polarized) and the reason is false (polarization is not exclusive to light waves), we conclude that the assertion is correct, but the reason is incorrect.

4. **Final Answer**:
- The correct option is that the assertion is true, but the reason is false.