Final answer:
The Soviet invasion that shifted America's foreign policy approach from neo-isolationism was the invasion of Afghanistan. This event re-engaged America in the global struggle against the spread of communism and away from the post-Vietnam war caution they were exercising. The correct option is 3.
Step-by-step explanation:
The Soviet invasion that jarred America from its post-Vietnam neo-isolationism was the invasion of Afghanistan. This significant event in cold war history rekindled American concerns about the spread of communism leading to a shift in the U.S. foreign policy. The conflict in Afghanistan served as a stark reminder that the geopolitical struggles between the superpowers of the United States and the Soviet Union were far from over.
Prompted by the Soviet action, President Jimmy Carter reacted by implementing measures that included the announcement of the Carter Doctrine and the boycotting of the 1980 Moscow Olympics.
Earlier, in the immediate aftermath of World War II, the U.S. encountered the Soviet Union in conflicts of interest regarding territories such as Iran and Turkey, and later during the Korean War, which was a critical moment of early Cold War tension between the superpowers. America's decision to intervene in these instances was driven by the containment policy, which aimed to prevent the spread of communism beyond the already established Iron Curtain.