Assertion : Kinematic equations are exact and are always correct.
Reason : The definitions of instantaneous velocity and acceleration are true only for motion in which the magnitude and direction of acceleration are constant during the course of motion.

Options

A. Both assertion and reason are true and reason is the correct explanation of assertion.
B. Both assertion and reason are true but reason is not the correct explanations of assertion.
C. Assertion is true but reason is false.
D. Both assertion and reason is false

Explanation

To solve the assertion and reason question, we will analyze both statements separately and determine their validity.

### Step 1: Analyze the Assertion
**Assertion:** Kinematic equations are exact and are always correct.

- Kinematic equations, such as \( v = u + at \), \( s = ut + \frac{1}{2}at^2 \), and \( v^2 = u^2 + 2as \), are derived under the assumption that acceleration is constant.
- If the acceleration is not constant, these equations do not hold true.
- Therefore, the assertion that kinematic equations are always correct is **false**.

### Step 2: Analyze the Reason
**Reason:** The definitions of instantaneous velocity and acceleration are true only for motion in which the magnitude and direction of acceleration are constant during the course of motion.

- Instantaneous velocity is defined as the limit of the average velocity as the time interval approaches zero, \( v = \frac{dx}{dt} \).
- Instantaneous acceleration is defined as the limit of the average acceleration as the time interval approaches zero, \( a = \frac{dv}{dt} \).
- These definitions are valid for any motion, regardless of whether the acceleration is constant or not.
- Thus, the reason is also **false**.

### Conclusion
Since both the assertion and the reason are false, the correct option is that both the assertion and reason are false.

### Final Answer:
Both assertion and reason are false.

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