The correct answer is option 2: First Amendment.

The Ninth Schedule was added to the Constitution of India through the First Amendment. The First Amendment was enacted in 1951 and introduced several changes to the original Constitution. One of the significant changes was the inclusion of the Ninth Schedule.

The Ninth Schedule provides protection to certain laws from judicial review. This means that laws included in the Ninth Schedule are shielded from being declared unconstitutional by the courts. It was introduced to safeguard land reform laws and other legislations related to social and economic reforms from legal challenges.

The inclusion of laws in the Ninth Schedule was initially intended to be a temporary measure. However, over the years, numerous other laws have been added to the schedule to protect them from judicial scrutiny. This has raised concerns about the misuse of the Ninth Schedule to protect laws that may be unconstitutional or against the principles of justice and equality.

In conclusion, the Ninth Schedule was added to the Constitution of India by the First Amendment in 1951, and it provides protection to certain laws from judicial review.