The correct answer is option 2: Constitutional Amendment. The voting age in India was lowered from 21 years to 18 years through a change made in the Indian Constitution. A Constitutional Amendment means making changes to the provisions of the Constitution. In this case, the amendment would have been passed by both houses of Parliament and would have required the support of at least two-thirds of the members present and voting, as well as the approval of more than half of the state legislatures. This is a significant process as it involves making changes to the fundamental law of the country. The other options - Presidential Ordinance, Executive Order, and Parliamentary Legislation - are not applicable in this context and are not the correct methods for changing the voting age in India.