Final answer:
During World War I, the majority of the trenches on the Western Front were built in France. Therefore, the correct option is a. France.
Step-by-step explanation:
During World War I, the majority of the trenches on the Western Front were built in France. This also meant that most of the fighting on the Western Front happened in France during the war. The trenches extended from the North Sea to the Swiss border, creating a network of opposing trenches that defined the conflict on the Western Front.