The correct answer is (b) 1971-81.
The population of Assam grew at a rate of 28.4% during the decade of 1971-81, which was the highest in its history. This was due to a number of factors, including a high birth rate, a low death rate, and an influx of refugees from Bangladesh.
The birth rate in Assam was 34.9 per 1,000 people in 1971, which was higher than the national average of 25.0 per 1,000 people. The death rate in Assam was 16.5 per 1,000 people in 1971, which was lower than the national average of 18.0 per 1,000 people. This meant that the natural increase in population (birth rate minus death rate) was 18.4 per 1,000 people in Assam in 1971, which was higher than the national average of 7.0 per 1,000 people.
In addition to the high natural increase, there was also an influx of refugees from Bangladesh into Assam during the 1971-81 decade. These refugees were fleeing the Bangladesh Liberation War, and they added to the population growth of Assam.
As a result of these factors, the population of Assam grew at a rate of 28.4% during the decade of 1971-81, which was the highest in its history.