The correct answer is:
D. Many people died, and it was the largest war in history because World War II hadn't happened yet.
When World War I was occurring, it was referred to as the "Great War" because it was the largest and most destructive conflict that had taken place up to that point in history. The term reflected its unprecedented scale and the significant loss of life. It was only later, after World War II occurred, that the term "World War I" was adopted to distinguish it from the subsequent global conflict.