By which Constitutional Amendment Act was the number of Lok sabha seats increased from 525 to 545 ?

The correct answer is Option D: Thirty-first Constitutional Amendment Act, 1973.

Let's break it down:
The question asks which amendment increased the number of Lok Sabha (India's lower house of Parliament) seats from 525 to 545.

Option A: The Twentieth Amendment Act, 1966 - This amendment dealt with validating certain judgments regarding the appointment of district judges in Uttar Pradesh. It's not related to the number of Lok Sabha seats.

Option B: The Forty-second Amendment Act, 1976 - Also known as the "Mini Constitution," this amendment made many changes to the Constitution, but it didn't directly change the number of Lok Sabha seats.

Option C: The Forty-fourth Amendment Act, 1978 - This amendment reversed some of the changes made by the 42nd Amendment and focused on restoring fundamental rights and limiting the power of the executive. It also didn't affect the number of Lok Sabha seats.

Option D: Thirty-first Constitutional Amendment Act, 1973 - This amendment specifically increased the number of Lok Sabha seats from 525 to 545. This was done to reflect the growing population of India.