The correct answer is: A. 1950
The provisions of citizenship in the Indian Constitution became applicable on January 26, 1950, the day India became a republic. The Constitution defines who is a citizen of India and sets out the rights and duties of citizens.
Option B is incorrect because the Constitution was adopted on November 26, 1949, but it did not become applicable until January 26, 1950.
Option C is incorrect because the Constitution was adopted
on November 26, 1949, and it became applicable on January 26, 1950, so it could not have become applicable in 1951.Option D is incorrect because the Constitution was adopted on November 26, 1949, and it became applicable on January 26, 1950, so it could not have become applicable in 1952.