Assertion : In potentiometer experiment, null point cannot be obtained if emf of unknown battery is more than the emf of known battery.
Reason : By increasing the emf of known battery, null point length increases.

Options

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

Answer

To solve the given question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understand the Assertion**:
- The assertion states that in a potentiometer experiment, a null point cannot be obtained if the EMF of the unknown battery (E1) is greater than the EMF of the known battery (E).
- This is based on the principle of the potentiometer, which states that the potential difference across a length of wire is proportional to the length of the wire. If E1 > E, there will not be enough potential difference in the potentiometer to balance the unknown EMF, hence no null point can be found.

2. **Understand the Reason**:
- The reason states that by increasing the EMF of the known battery, the null point length increases.
- This is misleading. While increasing the EMF of the known battery does increase the potential gradient, it does not necessarily mean that the null point length will increase. Instead, if the known battery's EMF is increased, the length at which the null point is found may actually decrease because the potential difference across the potentiometer wire becomes larger.

3. **Analyze the Relationship**:
- The potentiometer works on the principle that the potential gradient (V/L) is constant. If the EMF of the known battery is increased, the potential difference across the potentiometer wire increases, which can lead to a shorter length being required to balance the unknown EMF.
- Therefore, the assertion is correct, but the reason is incorrect.

4. **Conclusion**:
- Based on the analysis, we conclude that the assertion is true while the reason is false.

### Final Answer:
The assertion is true, but the reason is false.