Until the First World War, most countries in Europe were on the gold standard. Yet with the war, the system collapsed, because of the declining power of Great Britain and of the terrible economic conditions during and after the war. Even though there were some efforts to return to the safety of the gold standard in the second part of the 1920s, this did not last long due to the stock market crash, or the Great Depression, in late 1929. The Great Depression was the worst economic crisis in the history of the industrialized world, lasting from 1929 to roughly 1939. The correct answer is D.