The correct answer is C. Germany suffered the most casualties in World War I.
As we can see in the graph, Germany lost around 1.8 million casualties, with Russia in the 2nd place very close to Germany with near 1.7 million casualties.
A is false, the Allied Powers combined lost more casualties than the Central Powers. B is also false, since France lost near 1.4 million casualties and Austria-Hungary lost about 1.2 million people. D is false as well, Britain and France combined overpass 2 million casualties, while Germany has around 1.8 million victims.