Question

The arrival of which ally in 1917 was a turning point in the war because, it stopped the stalemate on the Western Front? *
Russia
United States
Ottoman Empire
France

Answer 1

It would be the arrival of the "United States" in 1917 that was a turning point in the war (World War I) because it stopped the stalemate on the Western Front, since the US provided essential troops and war supplies. 

Answer 2

The arrival of the United States in 1917 was a turning point in the war because, it stopped the stalemate on the Western Front