The correct answer is: b) 1950.
The Indian Constitution was adopted on 26 November 1949 and came into effect on 26 January 1950. It is the supreme law of India. The Constitution provides for a parliamentary system of government with a President as the head of state and a Prime Minister as the head of government. The Constitution also guarantees certain fundamental rights to all citizens of India.
Option a) is incorrect because India gained independence from British rule on 15 August 1947. However, the Indian Constitution was not adopted until 26 November 1949.
Option c) is incorrect because the Indian Constitution was adopted in 1949, not 1956.
Option d) is incorrect because the Indian Constitution was adopted in 1949, not 1975.