Which of the following diazonium salt is most stable ?

Options

A. `p-`Nitrobenzenediazonium chloride
B. `2, 4-`Dinitrobenzenediazonium chloride
C. `2, 4, 6-`Trinitrobenzenediazonium chloride
D. `p-`Methoxybenzenediazonium chloride

Explanation

To determine which diazonium salt is the most stable among the given options, we need to analyze the structures and the effects of the substituents on the stability of the diazonium salts.

### Step-by-Step Solution:

1. **Identify the Structures of the Diazonium Salts**:
- **Option 1**: Para-Nitrobenzene Diazonium Chloride (C6H4(NO2)N2Cl)
- **Option 2**: 2,4-Dinitrobenzene Diazonium Chloride (C6H3(NO2)2N2Cl)
- **Option 3**: 2,4,6-Tri-Nitrobenzene Diazonium Chloride (C6H2(NO2)3N2Cl)
- **Option 4**: Para-Methoxybenzene Diazonium Chloride (C6H4(OCH3)N2Cl)

2. **Analyze the Effects of Substituents**:
- The **nitro group (NO2)** is an electron-withdrawing group, which destabilizes the diazonium salt by increasing the positive charge on the nitrogen.
- The **methoxy group (OCH3)** is an electron-donating group, which stabilizes the diazonium salt by dispersing the positive charge.

3. **Evaluate Stability Based on Substituents**:
- **Option 1** (Para-Nitrobenzene): Contains one NO2 group, which will destabilize the salt.
- **Option 2** (2,4-Dinitrobenzene): Contains two NO2 groups, leading to greater destabilization than option 1.
- **Option 3** (2,4,6-Tri-Nitrobenzene): Contains three NO2 groups, resulting in the highest level of destabilization.
- **Option 4** (Para-Methoxybenzene): Contains one OCH3 group, which will stabilize the diazonium salt due to its electron-donating effect.

4. **Conclusion**:
- Since the stability of diazonium salts is enhanced by electron-donating groups and reduced by electron-withdrawing groups, the most stable diazonium salt among the options provided is **Para-Methoxybenzene Diazonium Chloride** (Option 4).

### Final Answer:
**The most stable diazonium salt is Para-Methoxybenzene Diazonium Chloride.**