(A):Acceleration is the rate of change of velocity .
(R ):A body having nonzero acceleration can have a constant velocity.

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 "Acceleration is the rate of change of velocity."
- According to physics, acceleration (A) is defined mathematically as:
\[ A = \frac{dv}{dt} \]
where \( dv \) is the change in velocity and \( dt \) is the change in time. This statement is true.

2. **Understanding the Reason**:
- The reason states that "A body having non-zero acceleration can have a constant velocity."
- If a body has a non-zero acceleration, it means that its velocity is changing over time. Therefore, it cannot maintain a constant velocity. This statement is false.

3. **Conclusion**:
- Since the assertion is true and the reason is false, we can conclude that the assertion is correct, but the reason does not support it.

### Final Answer:
- The assertion is true, and the reason is false.