The correct option is 1951-1981.
Explanation
India's demographic history is generally classified into four distinct phases of population growth based on trends in birth rates, death rates, and the overall rate of natural increase. These phases reflect the country's transition through different stages of demographic transition.
Analysis of Demographic Phases
- Phase I (1901-1921): Referred to as the period of stagnant or stationary phase. During this time, growth was extremely low, and in the decade 1911-1921, the growth rate was actually negative. Both birth rates and death rates were high due to poor health and medical services.
- Phase II (1921-1951): Known as the period of steady population growth. Improvements in health and sanitation reduced the mortality rate, while the crude birth rate remained high, leading to consistent growth.
- Phase III (1951-1981): This period is specifically referred to as the period of population explosion in India. This was caused by a rapid fall in the mortality rate due to developmental activities and improved medical facilities, while the fertility rate remained very high. The average annual growth rate was as high as 2.2%.
- Phase IV (Post-1981): This phase is characterized by high but declining growth rates. While the population continued to grow, the rate of growth started slowing down due to a downward trend in the crude birth rate.
Key Takeaway: The decades between 1951 and 1981 witnessed a "population explosion" because the gap between falling death rates and consistently high birth rates widened significantly, resulting in a sharp increase in population numbers.