Answer: B
Explanation: Declaring that the Old World and New World had different systems and must remain distinct spheres, Monroe made four basic points: (1) the United States would not interfere in the internal affairs of or the wars between European powers; (2) the United States recognized and would not interfere with existing colonies and dependencies in the Western Hemisphere; (3) the Western Hemisphere was closed to future colonization; and (4) any attempt by a European power to oppress or control any nation in the Western Hemisphere would be viewed as a hostile act against the United States. (source: Brittanica)
TL;DR: The United States basically said "hey, we're the bosses of the western hemisphere. If you mess with any of this, we'll pop a cap in your--."