The correct answer is option 3: 1950. The Constitution of India came into force on January 26, 1950. This was a significant milestone in the history of independent India, as it marked the country`s transition from being a dominion under the British monarchy to a sovereign republic.

Option 1: 1951 - This is incorrect. The Constitution was adopted in 1949 and came into force in 1950, not 1951.

Option 2: 1956 - This is incorrect. The year 1956 marks the adoption of the States Reorganization Act, which led to the reorganization of states in India. It is not related to the Constitution coming into force.

Option 4: 1949 - This is partially correct. The Constitution of India was adopted on November 26, 1949. However, it officially came into force on January 26, 1950.