Final answer:
The main legacy of World War I on Western society was a sense of disillusionment with pre-war institutions, as it ended the old order of monarchy and engendered skepticism toward republican democracy and the Enlightenment belief in rational progress.
Step-by-step explanation:
The main legacy of World War I on Western society was C, a sense of disillusionment with pre-war institutions. While the Enlightenment had promised the triumph of reason and progress, the horrific reality of World War I led many to question the very foundations of their societies. This cataclysmic event resulted in a definitive end to the old order of monarchy and nobility and sowed seeds of doubt about the efficacy of republican democracy, which had failed to prevent the war.
The war also sparked significant intellectual and cultural changes, as many intellectuals experienced a collapse in confidence in the culture of rationality that had been dominant since the Enlightenment. The post-war period was marked by political and economic turbulence, including the rise of fascism and totalitarian regimes, further disconnecting the pre-war belief in progress. The once-promised lasting peace and economic gains were overshadowed by the disillusionment permeating Western society, leading to a period of introspection and the eventual rise of different political movements and ideologies.