The main motivation for the formation of the Dominion of Canada was colonists were fearful of American invasion following the War for Independence.
In the 19th century, the British North American colonies that would later become Canada were scattered and vulnerable. The United States having gained its independence in the American Revolution posed a potential threat to these British colonies.
There was a genuine fear among colonists that the United States might seek to expand northward and annex British North American territories. To address this concern and to promote greater unity and cooperation among these colonies, the idea of confederation was proposed.
The British North America Act of 1867 which established the Dominion of Canada was motivated by the desire to provide a stronger defense against possible American expansionism and to ensure the security and stability of the British colonies in North America.
Therefore, the Option B is correct.