Correct one is

Options

A. `sigma_("semiconductor") gt sigma_("insulator") gt sigma_("metals")`
B. `sigma_("metal") gt sigma_("semiconductor") gt sigma_("insulator")`
C. `sigma_("semiconductor") gt sigma_("metals") gt sigma_("insulator")`
D. `sigma_("insulator") gt sigma_("semiconductor") gt sigma_("metals")`

Explanation

To solve the question regarding the conductivity of metals, semiconductors, and insulators, we need to analyze the properties of each material based on their electronic structure and energy band theory.

### Step-by-Step Solution:

1. **Understanding Conductivity (σ)**:
- Conductivity (σ) is defined as the ability of a material to conduct electric current. It is the inverse of resistivity (ρ), which means that higher conductivity corresponds to lower resistivity.

2. **Conductivity in Metals**:
- Metals have a very small or no energy gap between the conduction band (CB) and valence band (VB). This allows free electrons to move easily even at low temperatures.
- Therefore, the conductivity of metals is very high.

3. **Conductivity in Insulators**:
- Insulators have a large energy gap (greater than 3 eV) between the conduction band and valence band. This means that at room temperature, very few electrons can gain enough energy to jump from the valence band to the conduction band.
- As a result, insulators have very low conductivity.

4. **Conductivity in Semiconductors**:
- Semiconductors have a smaller energy gap (typically less than 3 eV). This allows some electrons to jump from the valence band to the conduction band with a small amount of thermal energy.
- The conductivity of intrinsic semiconductors is higher than that of insulators but lower than that of metals. Additionally, doping semiconductors can further increase their conductivity.

5. **Comparison of Conductivities**:
- Based on the above analysis, we can rank the conductivities:
- **Metals**: Highest conductivity (σ_metal > σ_semiconductor)
- **Semiconductors**: Intermediate conductivity (σ_semiconductor > σ_insulator)
- **Insulators**: Lowest conductivity (σ_insulator)

6. **Conclusion**:
- The correct order of conductivity is:
- σ_metal > σ_semiconductor > σ_insulator
- Therefore, the statement that "the correct one is σ_semiconductor and σ_insulator and σ_metal" is incorrect. The correct order is metals having the highest conductivity, followed by semiconductors, and then insulators.

### Final Answer:
The correct order of conductivity is:
**σ_metal > σ_semiconductor > σ_insulator**.