Assertion : Due to Frenkel defect, density of the crystalline solid decreases.
Reason : In Frenkel defect, cation or anion leaves the crystal.

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. If both assertion and reason are false.

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Due to Frenkel defect, density of the crystalline solid decreases." We need to evaluate this statement based on our understanding of Frenkel defects.

2. **Defining Frenkel Defect**: A Frenkel defect occurs when an ion (cation or anion) leaves its normal position in the crystal lattice and occupies an interstitial site. This means that while there is a vacancy where the ion was originally located, the ion itself is still present within the crystal structure, just in a different position.

3. **Analyzing Density**: Density is defined as mass per unit volume (Density = Mass/Volume). In the case of a Frenkel defect:
- The total mass of the crystal remains the same because the ion has not left the crystal; it has merely moved to a different location.
- The volume of the crystal also remains unchanged because the total number of ions in the crystal has not changed.

4. **Conclusion on Assertion**: Since both mass and volume remain unchanged, the density of the crystalline solid does not decrease. Therefore, the assertion is **false**.

5. **Understanding the Reason**: The reason states that "In Frenkel defect, cation or anion leaves the crystal." This is incorrect because, as explained, the ion does not leave the crystal; it simply relocates to an interstitial site.

6. **Conclusion on Reason**: Since the reason is also incorrect, we conclude that the reason is **false**.

7. **Final Evaluation**: Both the assertion and the reason are false. Therefore, the correct answer to the question is that both the assertion and reason are false.

### Final Answer:
Both assertion and reason are false.

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