A : In esterification reaction alcohol act as nucleophile .
R : In this reaction O - H bond of alcohol is broken .

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false , then mark (3)
D. If both Assertion and Reason are false statements , then mark (4)

Answer

To solve the question regarding the assertion and reason related to esterification reactions, we can break down the process into clear steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that in the esterification reaction, alcohol acts as a nucleophile.
- A nucleophile is a species that donates an electron pair to form a chemical bond in a reaction.

2. **Understanding the Reaction**:
- In the esterification reaction, a carboxylic acid reacts with an alcohol in the presence of an acid catalyst to form an ester and water.
- The general reaction can be represented as:
\[ \text{RCOOH} + \text{R'OH} \rightarrow \text{RCOOR'} + \text{H}_2\text{O} \]

3. **Mechanism of the Reaction**:
- The carboxylic acid (RCOOH) first gets protonated by the acid catalyst, increasing the electrophilicity of the carbonyl carbon.
- The alcohol (R'OH) then acts as a nucleophile, attacking the positively charged carbon atom of the carboxylic acid.

4. **Breaking of O-H Bond**:
- As the alcohol attacks the carbon, the O-H bond in the alcohol is broken to stabilize the positive charge that forms on the oxygen atom during the reaction.
- This bond breaking is essential for the alcohol to effectively act as a nucleophile.

5. **Conclusion**:
- Both the assertion and reason are true. The assertion correctly states that alcohol acts as a nucleophile, and the reason correctly explains that the O-H bond of the alcohol is broken during the reaction, allowing it to act as a nucleophile.
- Therefore, the correct option is that both the assertion and reason are true, and the reason explains the assertion.

### Final Answer:
- Both Assertion (A) and Reason (R) are true, and R explains A. Thus, the correct option is **Option 1**.