Pick up the correct statement:
(a) The crystal of AgBr does not have Schottky defect
(b) The crystal system having crystal parameters, `a = b != c, alpha= beta= 90^(@), gamma= 120^(@)` is for a hexagonal unit cell
(c) Ionic compounds having Frenkel defect, generally show a low radius ratio value.
(d) The coordination number of `Na^(+)` ion in NaCl is 4

Options

A. (a), (b)
B. (b), (c)
C. (c), (d)
D. (a), (c)

Answer

To solve the question, we need to evaluate each of the provided statements one by one to determine which one is correct.

### Step 1: Evaluate Statement (a)
**Statement (a)**: The crystal of AgBr does not have Schottky defect.
- **Analysis**: This statement is incorrect because AgBr can exhibit both Schottky and Frenkel defects. Schottky defect occurs when equal numbers of cations and anions are missing from the lattice, which can indeed happen in AgBr.
- **Conclusion**: Statement (a) is **incorrect**.

**Hint**: Remember that Schottky defects involve missing equal numbers of cations and anions.

### Step 2: Evaluate Statement (b)
**Statement (b)**: The crystal system having crystal parameters, `a = b != c`, `alpha = beta = 90°`, `gamma = 120°` is for a hexagonal unit cell.
- **Analysis**: This statement is correct. In a hexagonal unit cell, two sides (a and b) are equal, while the third side (c) is different. The angles α and β are both 90°, and γ is 120°, which matches the description of a hexagonal unit cell.
- **Conclusion**: Statement (b) is **correct**.

**Hint**: Recall the definitions of crystal systems and their parameters.

### Step 3: Evaluate Statement (c)
**Statement (c)**: Ionic compounds having Frenkel defect generally show a low radius ratio value.
- **Analysis**: This statement is also correct. Frenkel defects typically occur in ionic compounds where the size difference between cations and anions is small, leading to a low radius ratio. This is because the cation can fit into an interstitial site without much distortion of the lattice.
- **Conclusion**: Statement (c) is **correct**.

**Hint**: Consider the relationship between ionic size and defect types.

### Step 4: Evaluate Statement (d)
**Statement (d)**: The coordination number of `Na^+` ion in NaCl is 4.
- **Analysis**: This statement is incorrect. In NaCl, both Na^+ and Cl^- ions have a coordination number of 6, as they are arranged in a face-centered cubic (FCC) lattice.
- **Conclusion**: Statement (d) is **incorrect**.

**Hint**: Coordination number refers to the number of nearest neighbors surrounding an ion in the lattice.

### Final Conclusion
After evaluating all statements:
- Statement (a) is incorrect.
- Statement (b) is correct.
- Statement (c) is correct.
- Statement (d) is incorrect.

The correct statements are (b) and (c). However, since the question asks for the correct statement and typically expects a single answer, we can conclude that the most straightforward correct answer is **(b)**.

### Answer
The correct statement is **(b)**.