Assertion: Linear momentum of a planet does not remain conserved.
Reason: Gravitational force acts on it.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. f both assertion and reason are false.

Answer

To analyze the assertion and reason provided in the question, we can break down the concepts of linear momentum and the forces acting on a planet.

### Step-by-Step Solution:

1. **Understanding Linear Momentum**:
- Linear momentum (p) of an object is given by the product of its mass (m) and its velocity (v):
\[ p = mv \]
- For a system of particles, the total linear momentum is the vector sum of the individual momenta.

2. **Condition for Conservation of Linear Momentum**:
- Linear momentum of a system is conserved if there are no external forces acting on it. This means that if the net external force (F_net) is zero, then the total momentum of the system remains constant:
\[ F_{\text{net}} = 0 \implies \frac{dp}{dt} = 0 \implies p = \text{constant} \]

3. **Gravitational Force Acting on a Planet**:
- A planet, such as Earth, revolves around the Sun due to the gravitational force exerted by the Sun. This force is always directed towards the Sun and changes the direction of the planet's velocity, thus affecting its momentum.
- The gravitational force can be expressed as:
\[ F = \frac{G m_1 m_2}{r^2} \]
- Here, \(G\) is the gravitational constant, \(m_1\) is the mass of the Sun, \(m_2\) is the mass of the planet, and \(r\) is the distance between the centers of the two bodies.

4. **Conclusion on Momentum Conservation**:
- Since the gravitational force is acting on the planet, the net external force is not zero. Therefore, the linear momentum of the planet does not remain conserved.
- This supports the assertion that the linear momentum of a planet does not remain conserved.

5. **Evaluating the Reason**:
- The reason provided states that "Gravitational force acts on it." This is indeed a correct explanation for why the linear momentum of the planet is not conserved, as the presence of this force means that the conditions for momentum conservation are violated.

### Final Answer:
- The assertion is true, and the reason is a correct explanation for the assertion. Therefore, both the assertion and reason are correct.