a. The Treaty of Versailles
b. World War I
c. The Paris Peace Conference
d. The League of Nations
The correct answer is b. World War I.
The German occupation of Alsace and Lorraine was a major cause of World War I. The two provinces had been part of France since the 17th century, but were annexed by Germany in 1871 after the Franco-Prussian War. The French never forgot their lost territories, and the desire to regain them was a major factor in the outbreak of World War I.