In what regions did American participation in World War 1 have the greatest effect A. The Eastern Front B. The border of France and Belgium C. Africa D. Austria?

Answer 1

On the border of France and Belgium.
Americans were, by and large, not directly involved in the Eastern Front, where the Germans and Austrians fought against the Russians and knocked them out of the war. There was some convoying of supplies to Russia, as was done on a greater scale during WWII, but this was not a significant American effort. Africa was a colonial battle ground and did not receive significant American attention. Austria was a major belligerant, but fought mostly against Russia and Italy. American involvement was minimal.

The bulk of US troops in WW1 were located on the Western Front in France and Belgium and this is where American participation had its greates direct effect. That was the addition of fresh fighting units when both sides were strained after years of largely unproductive but massive and intense battles.
The Western Front

The border of France and Belgium

France