Concept:The League of Nations was formed after the First World War and existed before the Second World War began.Explanation:The League of Nations was created in 1920 following the First World War (1914−1918).Its main goal was to maintain world peace and prevent another major conflict.However, the League proved weak and could not stop the Second World War from breaking out in 1939.That war lasted until 1945.The other organizations came into existence much later.The UNO was founded in 1945, the OAU in 1963, and ECOWAS in 1975.Thus, only the League of Nations was in existence before the Second World War (1939−1945).Answer:C. The League of Nations