Assertion`:-` A particle having zero acceleration must have constant speed.
Reason `:-` A Particle having constant speed must have zero acceleration.

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion.
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertiion.
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are False

Explanation

To solve the question regarding the assertion and reason given, we will analyze both statements step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "A particle having zero acceleration must have constant speed."
- Acceleration is defined as the rate of change of velocity with respect to time, mathematically given by \( a = \frac{dv}{dt} \).
- If acceleration \( a = 0 \), it implies that \( \frac{dv}{dt} = 0 \). This means that the velocity \( v \) does not change over time.
- Therefore, if there is no change in velocity, the particle must be moving at a constant speed.
- **Conclusion**: The assertion is true.

2. **Understanding the Reason**:
- The reason states that "A particle having constant speed must have zero acceleration."
- Constant speed means that the magnitude of the velocity is constant; however, velocity is a vector quantity and includes both magnitude and direction.
- A classic example is uniform circular motion, where a particle moves with constant speed but changes direction continuously.
- In this case, even though the speed is constant, the direction of the velocity vector changes, which means there is an acceleration (centripetal acceleration) acting on the particle.
- Therefore, it is not accurate to say that constant speed implies zero acceleration.
- **Conclusion**: The reason is false.

3. **Final Evaluation**:
- Since the assertion is true and the reason is false, the correct answer is that the assertion is true but the reason is false.

### Final Answer:
- The correct option is: Assertion is true, Reason is false (Option C).