A : Longitudinal waves do not exhibit the phenomenon of polarisation .
R : In longitudinal waves medium particle vibrate in direction normal to the wave propagation .

Answer

To solve the given question, we need to analyze the assertion and the reason provided, and determine their truth values.

### Step 1: Analyze the Assertion
The assertion states that "Longitudinal waves do not exhibit the phenomenon of polarization."

- **Explanation**: This statement is true. Polarization is a phenomenon that occurs in transverse waves, where the oscillations are perpendicular to the direction of wave propagation. Longitudinal waves, such as sound waves, have oscillations that occur in the same direction as the wave propagation, and thus they cannot be polarized.

### Step 2: Analyze the Reason
The reason states that "In longitudinal waves, medium particles vibrate in the direction normal to the wave propagation."

- **Explanation**: This statement is false. In longitudinal waves, the particles of the medium vibrate in the same direction as the wave propagation (parallel to the direction of the wave). Therefore, the assertion is true, but the reason is incorrect.

### Step 3: Conclusion
Based on the analysis:
- The assertion is true.
- The reason is false.

Thus, the correct option is that the assertion is true, but the reason is false.

### Final Answer:
The correct option is C: Assertion is true, but Reason is false.

---