Final answer:
The significant root cause of World War I was nationalism, which led to increased national pride, opposition to foreign rule, and intensified rivalries between the Great Powers.
Step-by-step explanation:
One of the significant root causes of World War I was nationalism. Nationalism greatly influenced peoples and countries in the early 20th century, as it encouraged a sense of pride and unity within nations and also drove the desire for nations to assert their power and independence. This often led to opposition to foreign rule and increased rivalries between the major powers of the time. The other answer choices, such as the Treaty of Versailles, the Great Depression, and the League of Nations, are related to the aftermath of World War I or to events that occurred after the war and not its root causes. The acronym MAIN is useful for summarizing the long-term causes of World War I, which include Militarism, Alliances, Imperialism, and Nationalism.