Which of the following does not conduct electricity?

Options

A. Fused NaC1
B. Solid NaC1
C. Birne solution
D. copper

Explanation

To determine which of the following does not conduct electricity, we need to analyze the properties of the substances in question. The key factor for electrical conductivity is the presence of mobile ions or free electrons that can move in response to an electric field.

### Step-by-Step Solution:

1. **Understanding Electrical Conductivity**:
- Electrical conductivity in substances is primarily due to the movement of charged particles (ions or electrons).
- In solids, if the charged particles are fixed in place and cannot move, the substance will not conduct electricity.

2. **Analyzing Solid NaCl**:
- Sodium chloride (NaCl) in its solid form consists of a rigid lattice structure where Na+ and Cl- ions are held tightly together by ionic bonds.
- In this solid state, the ions are not free to move; hence, solid NaCl does not conduct electricity.

3. **Analyzing Fused (Molten) NaCl**:
- When NaCl is melted (fused), the rigid lattice breaks down, allowing Na+ and Cl- ions to move freely.
- In this molten state, NaCl can conduct electricity due to the presence of free-moving ions.

4. **Analyzing Brine Solution**:
- Brine is a solution of NaCl in water. When NaCl dissolves in water, it dissociates into Na+ and Cl- ions.
- These ions are free to move in the solution, allowing brine to conduct electricity.

5. **Analyzing Copper**:
- Copper is a metal that has free electrons in its structure. These free electrons can move easily, making copper a good conductor of electricity.

6. **Conclusion**:
- Among the substances analyzed, solid NaCl is the only one that does not conduct electricity due to the lack of mobile ions in its solid state.
- Therefore, the correct answer is solid NaCl.

### Final Answer:
**Solid NaCl does not conduct electricity.**