The groups which when present in para position tend to decrease the acidity of phenol are-

Options

A. `-NO_(2)`
B. `-CN`
C. `-OCH_(3)`
D. `-F`

Answer

To determine which groups, when present in the para position of phenol, decrease its acidity, we need to analyze the effects of various substituents on the acidity of phenol.

### Step-by-Step Solution:

1. **Understanding Phenol Structure**:
- Phenol has a hydroxyl group (-OH) attached to a benzene ring. The structure can be represented as:
\[ \text{C}_6\text{H}_5\text{OH} \]

2. **Acidity of Phenol**:
- The acidity of phenol is influenced by the substituents present on the benzene ring. The acidity is determined by the stability of the phenoxide ion (the conjugate base formed when phenol donates a proton).

3. **Electron-Withdrawing vs. Electron-Donating Groups**:
- **Electron-Withdrawing Groups (EWGs)**: These groups stabilize the negative charge on the phenoxide ion, increasing acidity. They typically exhibit a -I (inductive) or -R (resonance) effect.
- **Electron-Donating Groups (EDGs)**: These groups destabilize the negative charge on the phenoxide ion, decreasing acidity. They typically exhibit a +I (inductive) or +R (resonance) effect.

4. **Analyzing the Given Groups**:
- **NO2 (Nitro group)**: This is an electron-withdrawing group (-I and -R effect). It increases acidity. (Not our answer)
- **CN (Cyano group)**: This is also an electron-withdrawing group (-I and -R effect). It increases acidity. (Not our answer)
- **OCS3 (Methoxy group)**: This group has a lone pair on oxygen, which contributes a +R effect, making it an electron-donating group. It decreases acidity. (This is our answer)
- **Fluorine (F)**: Although it is highly electronegative and has a -I effect, it also has a +R effect due to resonance. However, its overall effect is still electron-withdrawing, thus increasing acidity. (Not our answer)

5. **Conclusion**:
- The group that decreases the acidity of phenol when present in the para position is **OCS3 (Methoxy group)**.

### Final Answer:
The groups which, when present in para position, tend to decrease the acidity of phenol are **OCS3 (Methoxy group)**.