Final answer:
World War I is considered a total war due to the full mobilization of societies and economies, use of new military technologies, and enormous civilian and military casualties.
Step-by-step explanation:
The development that best illustrates why World War I is classified as a total war is the extensive mobilization of not just military forces but entire national economies and societies to support the war effort. During World War I, technological innovations such as tanks, airplanes, machine guns, and poison gas were employed, which significantly increased the scale and lethality of combat. The war demanded vast resources and led to enormous casualties among combatants and civilians alike. Nations implemented wartime economies, dedicating all their resources to the war effort, which reflected the concept of total war, where the distinction between combatants and non-combatants was blurred, as entire societies were engaged in the pursuit of the war's objectives.