Final answer:
The AEF engaged in its first serious fighting in 1918, turning back the Germans at Château-Thierry on the Western Front, and then advancing to Saint-Mihiel.
Step-by-step explanation:
The American Expeditionary Force (AEF) saw its first serious fighting in 1918. It helped turn back the German offensive at Château-Thierry on the Western Front east of Paris. The American troops then advanced to nearby Saint-Mihiel. Therefore, the correct answer is D) 1918 - Château-Thierry - Western Front - Saint-Mihiel.