To aid black ________ into white society, federal and state governments implemented many democratic reforms between the years 1865 and 1875, the Reconstruction era.

“Democratic reforms” in latter part of the sentence provides clue for the blank. Democratic reforms implies the principle of equality of rights, opportunity and treatment. This implies that during the Reconstruction Era, the government was working on equal rights for blacks as well as for whites i.e. conformity between these two races. 'Assimilation' (the cultural absorption of a minority group into the main cultural body) in (4) would be the cause for implementation of such reforms, thereby confirming (4) as the answer.