Explanation:

- Option 1: Russia and Serbia were allies in World War I but were not involved in the First Battle of the Somme.

- Option 2: United States and France did not fight together at the Somme; the United States joined the war later, in 1917.

- Option 3: Austria and Italy were on opposite sides of World War I, and neither fought together in the Battle of the Somme.

- Option 4: Britain and France is correct. The First Battle of the Somme in 1916 saw British and French forces fighting against Germany in one of the war's largest battles.