A : The area under acceleration - time graph is equal to velocity of object.
R : For an object moving with constant acceleration, position - time graph is a straight line.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
C. If Assertion is true statement but Reason is false, then mark (3).
D. If both Assertion and Reason are false statement, then mark (4).

Answer

To solve the question, we need to analyze the assertion (A) and the reason (R) provided:

**Assertion (A):** The area under the acceleration-time graph is equal to the velocity of the object.

**Reason (R):** For an object moving with constant acceleration, the position-time graph is a straight line.

### Step-by-Step Solution:

1. **Understanding the Assertion (A):**
- The assertion states that the area under the acceleration-time graph represents the velocity of the object.
- According to physics, acceleration (a) is defined as the rate of change of velocity (v) with respect to time (t), which can be expressed mathematically as:
\[ a = \frac{dv}{dt} \]
- If we integrate acceleration with respect to time, we obtain:
\[ \int a \, dt = v + C \]
- The integral of acceleration over time gives us the change in velocity, which means the area under the acceleration-time graph indeed represents the change in velocity. Therefore, the assertion is **true**.

2. **Understanding the Reason (R):**
- The reason states that for an object moving with constant acceleration, the position-time graph is a straight line.
- However, this statement is incorrect. When an object is moving with constant acceleration, its velocity changes over time, and thus the position-time graph is not a straight line but a curve (specifically, a parabola).
- The correct relationship is that if acceleration is constant, the velocity-time graph is a straight line, while the position-time graph is a quadratic curve. Therefore, the reason is **false**.

3. **Conclusion:**
- Since the assertion is true and the reason is false, the correct option is:
- **Option 3:** Assertion is true but reason is false.

### Final Answer:
The correct answer is **Option 3**.

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