`R - overset(O)overset(||)(C ) - OH overset([?])(to) R CH_(2) -OH`
Here reagent is

Options

A. `LiAIH_(4)`
B. `NaBH_(4)`
C. Both (1)& (2)
D. Red `P//HI`

Explanation

To solve the question of identifying the reagent that converts a carboxylic acid (R-C(=O)-OH) to an alcohol (R-CH2-OH), we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Reaction Type**:
The reaction involves the conversion of a carboxylic acid (R-C(=O)-OH) to an alcohol (R-CH2-OH). This is a reduction reaction, where the carboxylic acid is being reduced to an alcohol.

**Hint**: Recognize that the conversion of a carboxylic acid to an alcohol requires a reducing agent.

2. **List Possible Reagents**:
The options given are:
- LiAlH4 (Lithium Aluminium Hydride)
- NaBH4 (Sodium Borohydride)
- Both 1 and 2
- Red P / HI (Red Phosphorus and Hydroiodic Acid)

**Hint**: Consider the strength of the reducing agents listed and their typical reactions with carboxylic acids.

3. **Evaluate Each Reagent**:
- **LiAlH4**: This is a strong reducing agent capable of reducing carboxylic acids to alcohols. It can donate hydride ions (H-) effectively to the carbonyl carbon of the carboxylic acid.

- **NaBH4**: This is a milder reducing agent that primarily reduces aldehydes and ketones. It is not effective for reducing carboxylic acids due to their lower reactivity compared to aldehydes and ketones.

- **Both 1 and 2**: Since NaBH4 cannot reduce carboxylic acids, this option is not valid.

- **Red P / HI**: This combination can reduce carboxylic acids to alkanes rather than alcohols. Therefore, it is not suitable for this reaction.

**Hint**: Compare the reactivity of the reagents with carboxylic acids to determine their suitability.

4. **Conclusion**:
Based on the evaluations, the only suitable reagent for the reduction of a carboxylic acid to an alcohol is **LiAlH4**.

**Final Answer**: The reagent is **LiAlH4 (Lithium Aluminium Hydride)**.