Final answer:
The correct answer is option a) League of Nations.
Step-by-step explanation:
The most logical solution to counter the spread of dictatorship in Germany and the Soviet Union during the prelude to World War II would have been the League of Nations. It was the global intergovernmental organization established after World War I to facilitate good relations among countries and to punish aggression through the principle of collective security. However, it failed to prevent the aggression of expansionist powers which contributed to the outbreak of World War II.
After the war, the United Nations was created to promote international peace and to have a more effective system of collective security that would require only a majority vote on most resolutions and a two-thirds majority on issues of peace and security. In response to Soviet aggression, the North Atlantic Treaty Organization (NATO) was formed to protect Western countries through collective defense, committing mutual military assistance among its members.
Given that the question seems to imply a historical context prior to the United Nations' formation, and the European Union and NATO came later, 'a) League of Nations' would be most historically relevant. However, if considering the post-World War II period, 'b) United Nations' or 'd) NATO' could be considered more logical solutions depending on the specific nature of the aggression and the geopolitical context.