The correct answer is option 2: 1939. The Second World War began in the year 1939. Option 1 (1940), option 3 (1941), and option 4 (1942) are incorrect. One key event that marked the start of the war was the German invasion of Poland on September 1, 1939. This aggression by Germany led to a chain of events that resulted in the outbreak of a global conflict. It is important to note that the war did not involve only Germany - it eventually involved many other countries, including major powers such as the United Kingdom, the Soviet Union, and the United States. The war lasted until 1945 and had a far-reaching impact on the world, both socially and politically.