Explanation:

- Option 3: 1921 is known as the year of the demographic divide in India. This year marked a transition from a declining population to one that started showing signs of growth. The 1921 census recorded a drop in population, known as the 'Great Divide', due to high mortality rates and low birth rates.

- Option 1: 1901 refers to the first decennial census conducted in India. It was significant but not termed as a demographic divide.

- Option 2: 1911 was part of the pre-divide era where population growth was still minimal.

- Option 4: 1931 saw continued population growth following the changes observed in 1921.