The given reaction involves a carbohydrate derivative reacting with dimethyl sulfate (\text{Me}_2\text{SO}_4) in the presence of sodium hydroxide (NaOH), followed by acidic hydrolysis with hydrochloric acid (HCl) and water.

Step-by-Step Reaction Explanation:

1. The reaction begins with \text{Me}_2\text{SO}_4 acting as a methylating agent. This will methylate the hydroxyl groups available on the carbohydrate molecule.
2. NaOH facilitates the deprotonation of OH groups, making them more nucleophilic, which aids in the methylation process.
3. All primary and secondary alcohol groups in the carbohydrate are converted to methyl ethers, except for the ketal formation (acetal linkages) typically unaltered in neutral/basic conditions.
4. In the second step, acid hydrolysis (using HCl and H₂O) results in breaking down any acetal formed (if applicable), restoring any hemiacetal to aldehyde if initially present. This reaction, however, is aimed at simply methylating without altering the primary sugar structure significantly.

Therefore, the most significant structural change is the conversion of free OH groups into OCH₃.

Conclusion:

The major product is the full methylated derivative of the carbohydrate, preserving the ring structure but converting each available hydroxyl group to a methoxy group.

This corresponds to the given correct answer option.