Assertion : In Frenkel defect, density of the crystalline solid does not change.
Reason : In Frenkel defect, no cation or anion leaves the crystal.

Answer

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

### Step 1: Understand the Assertion
The assertion states that "In Frenkel defect, the density of the crystalline solid does not change."

- **Frenkel Defect**: This defect occurs when an atom (usually a cation) is displaced from its normal lattice position to an interstitial site. This means that while one atom is missing from its original position, another atom occupies a different position within the crystal structure.

### Step 2: Analyze the Reason
The reason states that "In Frenkel defect, no cation or anion leaves the crystal."

- In a Frenkel defect, the total number of atoms in the crystal remains the same because the atom that is displaced to an interstitial site does not leave the crystal lattice. It simply moves from one position to another within the crystal.

### Step 3: Conclusion
Now, let's combine our understanding of both the assertion and the reason:

- Since the total number of atoms remains constant (as stated in the reason), the mass of the crystalline solid does not change.
- Density is defined as mass per unit volume. Since the mass remains the same and the volume of the crystal does not change significantly (as the atom is just moving to an interstitial site), the density of the crystalline solid does not change.

### Final Answer
Both the assertion and the reason are correct, and the reason correctly explains the assertion.