Assertion (A): Pentan-2-one on reaction with `NaHSO_(3)` gives sodium bisulphite adduct product.
Reason (R ): `(C=O)` group of pentan-3-one is sterically hindered and nucleophile `SO_(3)^(2-)` is also of large size, so nucleophilic addition reaction does not occur.

Options

A. Both (A) and (R ) true and (R ) is the correct explanation of (A).
B. Both (A) and (R ) are true but (R ) is not correct explanation of (A).
C. (A) is true but (R ) is false.
D. (A) is false but (R ) is true.

Answer

To solve the given assertion and reason question, we need to analyze both statements carefully.

### Step 1: Analyze the Assertion
The assertion states that pentan-2-one reacts with sodium bisulfite to give a sodium bisulfite adduct product.

**Explanation:**
Pentan-2-one is a ketone with the structure:
```
CH3
|
C=O
|
CH2-CH2-CH3
```
When a ketone reacts with sodium bisulfite (NaHSO3), the nucleophile (sulfite ion, SO3^2-) can attack the carbonyl carbon (C=O) to form a bisulfite adduct. This reaction typically occurs with ketones and aldehydes.

### Step 2: Analyze the Reason
The reason states that the carbonyl group of pentan-3-one is sterically hindered, and the nucleophile SO3^2- is also of large size, preventing the nucleophilic addition reaction from occurring.

**Explanation:**
Pentan-3-one has the structure:
```
CH3
|
C=O
|
CH2-CH2-CH3
```
In this case, the carbonyl carbon is flanked by two ethyl groups, which create steric hindrance. The sulfite ion (SO3^2-) is indeed a relatively large nucleophile. The steric hindrance from the ethyl groups makes it difficult for the nucleophile to approach and attack the carbonyl carbon effectively.

### Conclusion
- The assertion is **incorrect** because pentan-2-one (not pentan-3-one) can react with sodium bisulfite to form the adduct.
- The reason is **correct** because the steric hindrance from the ethyl groups in pentan-3-one does prevent the nucleophilic addition.

### Final Answer
Thus, the correct answer to the question is:
- Assertion (A) is **false**.
- Reason (R) is **true**.

The correct option is **D**: Reason is correct, Assertion is wrong.