In the mid-1800s, Democrats believed that a policy of Manifest Destiny would boost the American economy and open up economic opportunities.
It meant the the idea that the United States is destined by God and its advocates to to expand its dominion and spread democracy and capitalism across the entire North American continent.
Beyond the idea of Manifest Destiny, the territorial expansion over the 19th century resulted not only in war with Mexico but as well as the dislocation & brutal mistreatment of Native American, Hispanic and other non-European occupants of the territories now being occupied by the United States.
However, the Democrats believed that a policy of Manifest Destiny would boost the American economy and open up economic opportunities.
Therefore, the Option C is correct.
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