The correct answer is D. Monroe Doctrine.
With concerns about Spain reclaiming the Western territories, President Monroe signed in 1823 four major points:
The U.S. would not get involved in European affairs, not interfere with existing European colonies, no other nation could start a new colony in the Western Hemisphere and finally, if a European nation interfered with a nation in the Western Hemisphere, the U.S. would treat as a hostile act against this nation.