The correct statement in the following is-

Options

A. The ionic crystal of AgBr has Schottky defect
B. The unit cell having crystal parameters, `a = b ne c, alpha = beta = 90^(@), gamma = 120^(@)` is hexagonal
C. In ionic compounds having Frenkel defect the ratio `r_(+)/r_(-)` is high
D. The coordination number of `Na^(+)` ion in NaCl is 4

Explanation

To solve the question, we need to analyze the statements provided in the video transcript regarding ionic crystals and their defects.

### Step-by-Step Solution:

1. **Identify the Type of Crystal and Defects**:
- The video mentions an ionic crystal of AgBr (AGVR seems to be a typo). It states that this crystal has a Scott's defect.
- **Hint**: Understand what Scott's defect is and how it relates to ionic crystals.

2. **Understand Unit Cell Parameters**:
- The transcript describes unit cell parameters for a hexagonal crystal system where \( A = B \neq C \), \( \alpha = \beta = 90^\circ \), and \( \gamma = 120^\circ \).
- **Hint**: Recall the characteristics of hexagonal unit cells and how they differ from cubic and tetragonal cells.

3. **Frenkel Defect Analysis**:
- The video states that in ionic compounds exhibiting a Frenkel defect, the ratio \( R^+ / R^- \) (where \( R^+ \) is the radius of cation and \( R^- \) is the radius of anion) is low. This is incorrect; it should be high.
- **Hint**: Review the definitions of Frenkel and Schottky defects and their implications on ionic radii.

4. **Coordination Number in NaCl**:
- The video claims that the coordination number of \( Na^+ \) in \( NaCl \) is 4, which is incorrect. The coordination number is actually 6.
- **Hint**: Remember the structure of NaCl and how coordination numbers are determined.

5. **Conclusion**:
- Based on the analysis, the correct statement is about the unit cell parameters of the hexagonal crystal system. The other statements regarding the Frenkel defect and coordination number are incorrect.

### Final Answer:
The correct statement is that the unit cell parameters for a hexagonal crystal system are \( A = B \neq C \), \( \alpha = \beta = 90^\circ \), and \( \gamma = 120^\circ \).