among the following four compounds
(i) phenol (ii) methylphenol
(iii) meta - nitrophenol (iv) para - nitrophenol
the acidity order is:

Options

A. `ii gtigtiiigtiv`
B. `ivgtiiigtigtii`
C. `iiigtivigtigtii`
D. `igtivgtiiigtii`

Explanation

To determine the acidity order of the given compounds (i) phenol, (ii) methylphenol, (iii) meta-nitrophenol, and (iv) para-nitrophenol, we need to analyze how the substituents on the benzene ring affect the stability of the conjugate base formed after deprotonation (loss of H⁺).

### Step-by-Step Solution:

1. **Understanding Acidity**:
- The acidity of a compound is determined by how easily it can lose a proton (H⁺). The more stable the conjugate base (the species that remains after losing H⁺), the stronger the acid.

2. **Identifying the Compounds**:
- (i) **Phenol**: Contains a hydroxyl (-OH) group attached to a benzene ring.
- (ii) **Methylphenol**: Contains a methyl (-CH₃) group and a hydroxyl (-OH) group on the benzene ring. The position of the methyl group can affect acidity, but we will consider it at the ortho position for this analysis.
- (iii) **Meta-nitrophenol**: Contains a nitro (-NO₂) group and a hydroxyl (-OH) group on the benzene ring.
- (iv) **Para-nitrophenol**: Similar to meta-nitrophenol but the nitro group is in the para position relative to the hydroxyl group.

3. **Analyzing the Stability of Conjugate Bases**:
- **Phenol**: When phenol loses H⁺, it forms phenoxide ion (C₆H₅O⁻). This ion is relatively stable due to resonance stabilization.
- **Methylphenol**: The methyl group is an electron-donating group (+I effect), which destabilizes the phenoxide ion by increasing electron density on the oxygen. Thus, methylphenol is less acidic than phenol.
- **Meta-nitrophenol**: The nitro group is an electron-withdrawing group (-M effect), which stabilizes the conjugate base through resonance. However, its effect is less pronounced compared to the para position.
- **Para-nitrophenol**: The nitro group is positioned to exert a strong -M effect, pulling electron density away from the oxygen in the conjugate base, making it more stable than the meta form.

4. **Comparing Acidity**:
- **Order of Acidity**:
- Para-nitrophenol (most acidic due to strong -M effect)
- Meta-nitrophenol (less acidic than para due to weaker -M effect)
- Phenol (less acidic than meta-nitrophenol)
- Methylphenol (least acidic due to +I effect of methyl group)

5. **Final Acidity Order**:
- The correct order of acidity is: **para-nitrophenol > meta-nitrophenol > phenol > methylphenol**.