The correct answer is: b) 1971 Bangladesh Liberation War.
The 1971 Bangladesh Liberation War was a major conflict in South Asia that resulted in the creation of the independent country of Bangladesh. The war began on March 25, 1971, when the Pakistani military launched a crackdown on Bengali nationalists in East Pakistan. The crackdown led to a mass exodus of refugees into India, and the war ended on December 16, 1971, with the surrender of the Pakistani military.
The war had a significant impact on the demographics of Assam. In the aftermath of the war, millions of Bengali refugees from Bangladesh settled in Assam. This influx of refugees led to an increase in the population of Assam, and also to tensions between the Bengali refugees and the indigenous Assamese people.
The options a), c), and d) are incorrect because they do not correspond to a period of significant in-migration from Bangladesh to Assam. The 1950s were a period of relative peace and stability in South Asia, and there was no major influx of refugees from Bangladesh during this time. The 1990s and 2000s were also periods of relative peace and stability, and there was no major influx of refugees from Bangladesh during these times either.