Which amendment to the Constitution of India changed the description of India from a 'sovereign democratic republic' to a 'sovereign, socialist secular democratic republic' and also changed the words 'unity of the nation' to 'unity and integrity of the nation'?

42nd

The correct answer is Option 3 - 42nd

The amendment that changed the description of India in the Constitution's Preamble  from "sovereign democratic republic" to "sovereign, socialist secular democratic republic" and "unity of the nation" to "unity and integrity of the nation" was the Forty-second Amendment Act of 1976.

The exact text of the Preamble - 

WE, THE PEOPLE OF INDIA having solemnly resolved to constitute India into a SOVEREIGN SOCIALIST SECULAR DEMOCRATIC REPUBLIC and to secure to all its citizens:

JUSTICE, social, economic and political;

LIBERTY of thought, expression, belief, faith and worship;

EQUALITY of status and of opportunity; and to promote among them all

FRATERNITY assuring the dignity of the individual and the unity and integrity of the Nation;

IN OUR CONSTITUENT ASSEMBLY this twenty-sixth day of November, 1949, do HEREBY ADOPT, ENACT AND GIVE TO OURSELVES THIS CONSTITUTION.