In which of the following molecules `pi`-electron density in ring is maximum.

Options

A.
B.
C.
D.

Explanation

To determine which molecule has the maximum π-electron density in the ring, we need to analyze the substituents on the benzene ring and their effects on electron density. The substituents can either donate or withdraw electron density through resonance and inductive effects.

### Step-by-Step Solution:

1. **Identify the Substituents**:
The question involves four different substituents on a benzene ring:
- Nitro group (NO2)
- Hydroxide ion (O^-)
- Amino group (NH2)
- Methoxy group (OCH3)

2. **Determine the Effects of Each Substituent**:
- **Nitro group (NO2)**: This group is a strong electron-withdrawing group due to its -I (inductive) and -M (mesomeric) effects. It decreases the electron density in the ring.
- **Hydroxide ion (O^-)**: This group is a strong electron-donating group due to its +I (inductive) and +M (mesomeric) effects. It increases the electron density in the ring.
- **Amino group (NH2)**: The amino group has a +M effect (donating electrons through resonance) and a -I effect (withdrawing electrons through induction). However, the +M effect dominates, leading to an increase in electron density.
- **Methoxy group (OCH3)**: Similar to the amino group, the methoxy group has a +M effect (donating electrons through resonance) and a -I effect (withdrawing electrons through induction). The +M effect is stronger, leading to an increase in electron density.

3. **Compare the Effects**:
- The nitro group decreases electron density.
- The hydroxide ion increases electron density significantly.
- The amino group also increases electron density, but not as strongly as the hydroxide ion.
- The methoxy group increases electron density, but again, not as strongly as the hydroxide ion.

4. **Conclusion**:
Among the substituents, the hydroxide ion (O^-) has the strongest electron-donating effect, leading to the highest π-electron density in the benzene ring. Therefore, the molecule with the maximum π-electron density in the ring is the one with the hydroxide ion (O^-).

### Final Answer:
The molecule with the maximum π-electron density in the ring is the one with the hydroxide ion (O^-).