The correct response is:
A. It gave Europe the message that the Western Hemisphere was no longer open for colonization.
The Monroe Doctrine, articulated in 1823 by President James Monroe, asserted that any further European colonization or intervention in the Americas would be viewed as acts of aggression, prompting U.S. intervention. The doctrine was primarily aimed at preventing European intervention in Latin America and showcasing the United States' intention to remain involved in Western Hemisphere affairs.