Assertion : If some ferromagnetic substance is filled inside a solenoid, its coefficient of self induction `L` will increase.
Reason : By increasing the current in a coil, its coefficient of self induction `L` can be increased.

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 explanation 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 assertion and reason question, we need to analyze both statements carefully.

### Step 1: Understand the Assertion
The assertion states that if a ferromagnetic substance is filled inside a solenoid, its coefficient of self-induction \( L \) will increase.

- **Explanation**: When a ferromagnetic material is introduced into a solenoid, it enhances the magnetic field inside the solenoid due to its high permeability. This leads to an increase in the magnetic flux \( \Phi \) through the solenoid. The self-inductance \( L \) of a solenoid is given by the formula:
\[ L = \frac{N \Phi}{I} \]
where \( N \) is the number of turns, \( \Phi \) is the magnetic flux, and \( I \) is the current. Since the introduction of the ferromagnetic material increases \( \Phi \), the self-inductance \( L \) will also increase.

### Step 2: Understand the Reason
The reason states that by increasing the current in a coil, its coefficient of self-induction \( L \) can be increased.

- **Explanation**: This statement is incorrect. The self-inductance \( L \) of a coil is a property of the coil itself and depends on its geometry and the material inside it, not on the current flowing through it. Increasing the current \( I \) does not change the self-inductance \( L \); it only affects the magnetic flux \( \Phi \) and the induced emf in the coil.

### Step 3: Conclusion
Based on the analysis:
- The assertion is **correct**: Filling a ferromagnetic substance inside a solenoid increases its self-inductance \( L \).
- The reason is **incorrect**: Increasing the current does not increase the self-inductance \( L \).

Thus, the final conclusion is:
- **Assertion**: True
- **Reason**: False

### Final Answer
- Assertion is correct, but the reason is incorrect.

---