The correct answer is option 2, the 42nd Amendment Act. The 42nd Amendment Act was passed in 1976 and brought about several changes in the Constitution of India. One of these changes was the insertion of the word `socialist` into the Preamble of the Constitution.

The Preamble of the Indian Constitution outlines the ideals and aspirations of the nation. It sets the tone for the constitution and represents the basic principles on which the country is founded. The original Preamble, adopted in 1950, described India as a "sovereign democratic republic."

The 42nd Amendment Act introduced the word `socialist` into the Preamble, along with the words `secular` and `integrity.` This change was made to reflect the commitment of the Indian state to the principles of socialism, secularism, and national unity.

It is important to note that the 42nd Amendment Act also made several other significant changes to the Constitution, some of which were controversial. The amendment expanded the powers of the executive and curtailed the powers of the judiciary. It also introduced fundamental duties for citizens. Overall, the 42nd Amendment Act represented a significant shift in the constitutional framework of India.