A : Gauch conformer of ethylene glycol is most stable
R : It is due to the formation of intramolecular hydrogen bonding:

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 assertion and reasoning question regarding the stability of the gauche conformer of ethylene glycol, we will analyze both the assertion (A) and the reason (R) step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that the gauche conformer of ethylene glycol is the most stable.
- Ethylene glycol has the molecular formula C₂H₆O₂, which can be represented as HO-CH₂-CH₂-OH.

2. **Identifying the Conformers**:
- In conformational analysis, we consider different spatial arrangements of the molecule due to rotation around single bonds.
- The two primary types of conformers for ethylene glycol are the **gauche** and **anti** conformers. The gauche conformer has the hydroxyl (OH) groups positioned at a dihedral angle of 60° relative to each other, while the anti conformer has them at 180°.

3. **Stability of Conformers**:
- The stability of conformers can be influenced by factors such as steric hindrance and hydrogen bonding.
- In the case of ethylene glycol, the gauche conformer is more stable than the anti conformer due to the presence of intramolecular hydrogen bonding between the hydroxyl groups.

4. **Understanding the Reason (R)**:
- The reason states that the stability of the gauche conformer is due to the formation of intramolecular hydrogen bonding.
- Intramolecular hydrogen bonding occurs when a hydrogen atom bonded to an electronegative atom (like oxygen in OH groups) interacts with another electronegative atom within the same molecule.

5. **Conclusion**:
- Both the assertion (A) and the reason (R) are true.
- The reason provided (intramolecular hydrogen bonding) correctly explains why the gauche conformer is more stable than the anti conformer.

### Final Answer:
- Both the assertion and the reason are true, and the reason is the correct explanation for the assertion.