In what section of the Western Front did the Americans see action in WWI?
The American Expeditionary Force saw action in the part of the Western Front that was farthest to the east. This was an area that was pretty far to the south, south of Luxembourg and more or less directly east of Paris.
There, the Americans participated in many important battles. They fought to reduce the Saint Mihiel salient. They fought at Bellau Wood. They fought at the battle of Meuse-Argonne. This last battle was the last major offensive of the war.
The Americans, then, fought in the sector of the Western Front that was farthest to the east.