The true statement about the American Expeditionary Force (AEF) is:
The AEF broke the stalemate in favor of the Allies.
The AEF, under the command of General John J. Pershing, played a critical role in several key offensives, including the Meuse-Argonne Offensive, which contributed significantly to the breaking of the stalemate on the Western Front and ultimately helped lead to the defeat of Germany in World War I.
The other statements are not accurate: while the AEF did arrive later in the war, it saw significant combat; it demonstrated effectiveness despite initial inexperience; and while coordination with Allied forces occurred, the AEF operated independently and didn't rely solely on British support.