What was the most important factor responsible for the decline in the…

The correct answer is Option 2: Control over epidemic diseases

Both famine control and disease control contributed to the decline in death rates after 1921. However, the NCERT explicitly states that control over epidemic diseases was perhaps the most important factor. Epidemics such as plague, smallpox, cholera, and especially the influenza pandemic of 1918–19 had caused massive mortality in India. Their gradual control played a crucial role in reducing death rates.

NCERT: "Before 1931, both death rates and birth rates were high, whereas, after this transitional moment the death rates fell sharply but the birth rate only fell slightly. The principal reasons for the decline in the death rate after 1921 were increased levels of control over famines and epidemic diseases. The latter cause was perhaps the most important. The major epidemic diseases in the past were fevers of various sorts, plague, smallpox and cholera. But the single biggest epidemic was the influenza epidemic of 1918-19, which killed as many as 170 lakh people, or about 5% of the total population of India at that time. (Estimates of deaths vary, and some are much higher. Also known as ‘Spanish Flu’, the influenza pandemic was a global phenomenon . "