The correct answer is option 3 - 1971.

The decadal growth rate of population is the percentage increase in population over a period of ten years. In this question, we are asked for the year in which India experienced the highest decadal growth rate of population.

To determine the answer, we need to look at the population data of India over the years. From historical records, it is known that India experienced a significant increase in population during the 20th century.

Looking at the options provided, we can see that the years mentioned represent the starting year of each decade. We need to find the year in which the population growth rate was the highest.

By analyzing the population data, it is known that the population growth rate was highest in India during the 1970s. Specifically, the highest decadal growth rate of population occurred between 1961 and 1971. Therefore, the correct answer is option 3 - 1971.

The other options (1991, 1981, 1961) do not represent the year with the highest decadal growth rate of population in India during the 20th century.