Which Canadian Prime Minister was in office during most of World War I?

Robert Borden was the Prime Minister of Canada during the majority of World War I, serving from 1911 to 1920. His government played a crucial role in mobilizing Canadian resources and troops for the war effort, and he was instrumental in the establishment of the Canadian Expeditionary Force. Borden's leadership was significant in navigating the challenges of wartime Canada, including implementing conscription in 1917, which was a contentious issue but essential for supplementing Canadian forces in Europe.

Understanding the timeline of Canada’s involvement in World War I highlights Borden's pivotal role. The war began in 1914, and Canada declared war on Germany shortly after Britain did, reflecting its status as a dominion within the British Empire. Throughout the war, Borden supported measures that expanded Canada's military contributions and also engaged in discussions that would eventually lead to the recognition of Canada's autonomy in international affairs post-war, exemplified by his presence at the Paris Peace Conference in 1919.

The other individuals listed were not in office during the relevant time frame. William Lyon Mackenzie King served primarily after World War I; Lester B. Pearson and John Diefenbaker were in office much later. This context underscores why Robert Borden is the correct answer