Correct option is A
The key message of the Monroe Doctrine is: (a) It warned the European powers against any attempt to extend their power in the western hemisphere.
Information Booster
The doctrine, articulated by U.S. President James Monroe in 1823, established two core principles regarding the Western Hemisphere (North and South America):
Non-Colonization: European powers were forbidden from establishing new colonies in the Americas.
Non-Intervention: Any attempt by a European power to interfere with, oppress, or control any nation in the Western Hemisphere would be viewed as an unfriendly act towards the United States.
In essence, it declared that the Americas were no longer open for European colonization or political manipulation.
Additional knowledge
(2) A blueprint of Dollar diplomacy: Incorrect. Dollar Diplomacy refers to the U.S. policy (early 20th century) of using its economic power to exert influence in Latin America, which is a later development and a form of U.S. intervention, arguably a departure from the Monroe Doctrine's original spirit.
(3) It abolished slavery from America: Incorrect. Slavery was abolished in the U.S. decades later with the Thirteenth Amendment in 1865.
(4) It was a doctrine of economic assistance to countries of Latin America: Incorrect. This sounds more like the Good Neighbor Policy (1930s) or post-World War II aid policies, not the Monroe Doctrine, which was primarily a political and territorial warning.