The correct answer is: a) Assam Accord.
The Assam Accord was a peace accord signed on 15 August 1985 between the Government of India and the All Assam Students Union (AASU) to resolve the Assam Movement. The accord led to the deportation of over 2 million illegal immigrants from Assam.
The Indo-Bangladesh Treaty was signed on 6 May 1974 between India and Bangladesh. The treaty defined the land boundary between the two countries and provided for the exchange of enclaves. The treaty did not play a role in the in-migration of refugees to Assam.
The Land Boundary Agreement was signed on 6 July 2015 between India and Bangladesh. The agreement exchanged enclaves and corrected the land boundary between the two countries. The agreement did not play a role in the in-migration of refugees to Assam.
The Teesta Water Sharing Agreement was signed on 6 December 2015 between India and Bangladesh. The agreement provides for the sharing of water from the Teesta River between the two countries. The agreement did not play a role in the in-migration of refugees to Assam.