The molecule that has minimum/no role in the formation of photochemical smog, is:

Options

A. NO
B. `CH_2=O`
C. `N_2`
D. `O_3`.

Explanation

To determine which molecule has minimum or no role in the formation of photochemical smog, we need to analyze the options provided and understand the role of each molecule in the formation of photochemical smog.

### Step-by-step Solution:

1. **Understanding Photochemical Smog**:
Photochemical smog is primarily formed when sunlight reacts with pollutants such as nitrogen oxides (NOx) and volatile organic compounds (VOCs). Key components include ozone (O3), nitrogen oxides (NO), and hydrocarbons.

2. **Analyzing the Options**:
- **NO (Nitric Oxide)**: This is a nitrogen oxide that plays a significant role in the formation of photochemical smog. It reacts with other compounds in the presence of sunlight to form ozone.
- **CH2O (Formaldehyde)**: This is a volatile organic compound (VOC) that also contributes to the formation of photochemical smog. It can react with other pollutants under sunlight to form secondary pollutants.
- **N2 (Nitrogen Gas)**: This is an inert gas that constitutes about 78% of the Earth's atmosphere. It does not react with other atmospheric components and does not contribute to the formation of photochemical smog.
- **O3 (Ozone)**: Ozone is a major component of photochemical smog and is formed through reactions involving nitrogen oxides and volatile organic compounds in the presence of sunlight.

3. **Conclusion**:
Based on the analysis, the molecule that has minimum or no role in the formation of photochemical smog is **N2 (Nitrogen Gas)**.

### Final Answer:
**N2 (Nitrogen Gas)**