Assertion : When a dielectric is placed in an electric field, the electric field with in the dielectric as well as. Potential difference across the capacitor plates are reduced by a factor K (battery is not connected).
Reason : The dielectric constant is the ratio of the absolute permittivity of the dielectric to the to the permittivity of the free space.

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 correct explanation of Assertion.
C. If Assertion is true but Reason is false.
D. If Assertion is false but Reason is true.

Answer

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

### Step 1: Understanding the Assertion
The assertion states that when a dielectric is placed in an electric field, the electric field within the dielectric and the potential difference across the capacitor plates are reduced by a factor \( K \) when the battery is not connected.

- When a dielectric material is inserted into a capacitor, it polarizes, which reduces the effective electric field (\( E \)) within the dielectric.
- The potential difference (\( V \)) across the capacitor plates is related to the electric field and the distance (\( d \)) between the plates by the formula:
\[ V = E \cdot d \]
- Since the electric field is reduced by a factor of \( K \), the potential difference will also be reduced by the same factor \( K \).

### Step 2: Understanding the Reason
The reason states that the dielectric constant (\( K \)) is the ratio of the absolute permittivity of the dielectric (\( \varepsilon \)) to the permittivity of free space (\( \varepsilon_0 \)):
\[ K = \frac{\varepsilon}{\varepsilon_0} \]
- This statement is correct as it defines the dielectric constant accurately.

### Step 3: Analyzing the Relationship
While both the assertion and the reason are correct, the reason does not provide a correct explanation for the assertion. The assertion discusses the effects of introducing a dielectric on the electric field and potential difference, while the reason merely defines the dielectric constant without linking it to the reduction of the electric field and potential difference.

### Conclusion
- Both the assertion and the reason are true statements.
- However, the reason does not correctly explain the assertion.

Thus, the final answer is that both the assertion and reason are correct, but the reason is not a correct explanation for the assertion.

### Final Answer
**Assertion is true, Reason is true, but Reason is not the correct explanation for Assertion.**