The correct answer is option 2: 1976. The Fundamental Duties of Indian citizens were incorporated into the Constitution of India in 1976 through the 42nd Amendment. These duties were added to the Part IV-A of the Constitution, which is known as the Directive Principles of State Policy.

Option 1 (1952) is incorrect as the Fundamental Duties were not incorporated into the Constitution at that time.

Option 3 (1979) is also incorrect as the Fundamental Duties were already incorporated by this year.

Option 4 (1981) is incorrect as the Fundamental Duties were already incorporated by this year.

Therefore, the correct answer is option 2: 1976.