The correct answer to why the First Battle of Marne was considered a turning point in World War I is:
C. It was the first engagement to use trench warfare.
Trench warfare became a defining feature of World War I, and the First Battle of Marne was significant as it marked the beginning of this brutal form of warfare. The battle demonstrated the effectiveness of trench warfare in halting the German advance towards Paris. By digging in and using trenches, the Allied forces were able to stop the German offensive, leading to a stalemate that would characterize much of the war on the Western Front. Trench warfare brought about a new era of warfare tactics and strategies, making the First Battle of Marne a pivotal moment in World War I.
