The Battle of the Somme correctly completes the sentence. Here's why:
1. The Battle of the Somme took place between July and November 1916 during World War I.
2. It was one of the largest battles of the war and is often remembered for the high number of casualties on both sides.
3. The Battle of the Somme was known for the first use of the tank in warfare and the British army's advance on the German lines.
4. Therefore, when completing the sentence "The Battle of [blank] was the longest battle of World War I," the correct option is "the Somme" because of the historical significance and duration of this particular battle.
