Assertion : In moving from centre of a solid sphere to its surface, gravitational potential increases.
Reason : Gravitational field strength increase.

Options

A. If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.
B. If both Assertion and Reason are true but Reason is not the correct explantion of Assertion.
C. If Assertion is true, but the Reason is false.
D. If Assertion is false but the Reason is true.

Explanation

To solve the question, we will analyze the assertion and the reason step by step.

### Step 1: Understand the Assertion
The assertion states that "In moving from the center of a solid sphere to its surface, gravitational potential increases."

- **Gravitational Potential (V)** is defined as the work done per unit mass to bring a mass from infinity to a point in the gravitational field. For a solid sphere, the gravitational potential at the center (Vc) is given by:
\[ V_c = -\frac{3GM}{2R} \]
where \( G \) is the gravitational constant, \( M \) is the mass of the sphere, and \( R \) is the radius of the sphere.

- The gravitational potential at the surface (Vs) is given by:
\[ V_s = -\frac{GM}{R} \]

- Comparing \( V_c \) and \( V_s \):
\[ V_c < V_s \]
This means that as we move from the center to the surface, the gravitational potential actually increases (becomes less negative).

### Step 2: Understand the Reason
The reason states that "Gravitational field strength increases."

- **Gravitational Field Strength (E)** is defined as the force per unit mass experienced by a small mass placed in the field. Inside a solid sphere, the gravitational field strength at the center is zero, and it increases as we move towards the surface.

- At the center of the sphere:
\[ E_c = 0 \]
- At the surface of the sphere:
\[ E_s = \frac{GM}{R^2} \]
- Thus, the gravitational field strength increases from zero at the center to a maximum value at the surface.

### Step 3: Conclusion
Both the assertion and the reason are correct statements. However, the reason does not correctly explain the assertion. The assertion is about the increase in gravitational potential, while the reason discusses the increase in gravitational field strength, which are related but not directly explanatory of each other.

### Final Answer
The correct option is that both statements are correct, but the reason is not the correct explanation of the assertion.