World War I significantly influenced modernist writing in various ways, but the most fitting description from your options is: Modernist writing expressed the changes in people's beliefs and attitudes due to the war.
The war led to a profound disillusionment with traditional values and norms, causing writers to explore themes of fragmentation, alienation, and the breakdown of societal structures. Modernist writers often depicted the psychological impacts of the war on individuals and society, reflecting a shift in perception and belief systems that emerged from the experiences and aftermath of the conflict. While some works did protest the war or depict it critically, the overall modernist movement is characterized more by its exploration of the complexities of human experience in a rapidly changing world, heavily influenced by the traumas of the war.