A : `CH_(3) - underset(CH_(3))underset(|)overset(CH_(3))overset(|)(C)-CH=CH_(2)` on hydroboration oxidation gives `CH_(3)-underset(OH)underset(|)overset(CH_(3))overset(|)(C)-underset(CH_(3))underset(|)(CH)-CH_(3)` as major product .
R : It involves the formation of carbocation so undergoes rearrangement .

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, we need to analyze the assertion (A) and reason (R) provided regarding the hydroboration-oxidation of the given compound.

### Step-by-Step Solution:

1. **Identify the Compound**:
The compound given is `CH₃-CH(CH₃)-C=CH₂`. This is a substituted alkene where the double bond is between carbon 3 and carbon 4.

2. **Understand Hydroboration-Oxidation**:
Hydroboration-oxidation is a two-step reaction:
- In the first step, borane (BH₃) adds across the double bond. This addition occurs in an anti-Markovnikov manner, meaning that the boron atom attaches to the less substituted carbon atom.
- In the second step, oxidation occurs with hydrogen peroxide (H₂O₂) in the presence of a base, converting the boron group into a hydroxyl (-OH) group.

3. **Apply Hydroboration**:
In our case, the double bond is between C3 and C4. The less substituted carbon is C4 (which has only one hydrogen), while C3 is more substituted (it has two methyl groups). Therefore, during hydroboration:
- Boron (B) will attach to C4.
- Hydrogen (H) will attach to C3.

The intermediate product after hydroboration will be:
`CH₃-CH(CH₃)-C(BH₂)-CH₂`

4. **Perform Oxidation**:
In the oxidation step, the boron atom will be replaced by a hydroxyl group. Thus, the product will be:
`CH₃-CH(CH₃)-C(OH)-CH₂`

5. **Final Product**:
The final product after hydroboration-oxidation is `CH₃-CH(CH₃)-C(OH)-CH₃`, which indicates that the hydroxyl group is indeed on the more substituted carbon (C3), not C1 as stated in the assertion.

6. **Evaluate Assertion and Reason**:
- **Assertion (A)**: The assertion states that the alcohol is substituted at the third carbon, which is incorrect because the hydroxyl group is actually on the second carbon.
- **Reason (R)**: The reason states that the reaction involves the formation of a carbocation, which is also incorrect because hydroboration does not involve carbocation formation; it proceeds via a concerted mechanism without forming intermediates.

### Conclusion:
Both the assertion and the reason are false. Therefore, the correct answer is that both A and R are false.