Final answer:
The correct answer is B) United Nations intervention. During the Korean War in the early 1950s, the action that occurred for the first time in history was the United Nations intervention.
Step-by-step explanation:
During the early 1950s, the Korean War saw the United Nations intervene for the first time in history. When North Korean troops invaded South Korea, the United States called on the UN for help.
The UN condemned the invasion and created a joint force of troops from fifteen nations, placing them under the command of Douglas MacArthur. This marked the first time that the UN had mobilized a multinational force to respond to an act of aggression.