The correct answer is B) because he knew that a unified American front could intimidate the Central Powers.
General John J. Pershing refused to allow the AEF to serve as "fill in the gaps" of soldiers in Europe because he knew that a unified American front could intimidate the Central Powers.
During World War 1, on September 12, 1918, the American Expeditionary Forces led by General John J. Pershing launched a major offensive. It was remarkable because the AEF was an independent Army. This means that until that day, the US armed forces had supported the British and the French army that already had positions on the battlefield, but Pershing and his men launched its own main campaign against the Germans.
The other options of the question were A) because he did not want to work with the Allied Powers, C) because he could not speak a foreign language, and D) because his soldiers requested to only serve the US during Selective Service.