Final answer:
The United States met the demand for soldiers after entering World War I through mandatory conscription by passing the Selective Service Act of 1917, a continuation of the practice used during the Civil War. The correct answer is option d.
Step-by-step explanation:
The United States met the demand for soldiers after entering World War I by implementing mandatory conscription. Initially, there was hope to rely on an all-volunteer army, but significant challenges made this impractical. First, Congress doubted the sufficiency of volunteer enlistment to significantly expand the size of the army from its existing numbers. Secondly, there was concern that volunteer enlistment would lead to a loss of men with vital industrial skills, who were more urgently needed on the home front to ensure the production and supply of war materials.
The situation called for a more reliable and systematic approach to raising an army. To address these concerns, Congress passed the Selective Service Act of 1917, introducing a peacetime draft. Military conscription was not a novel idea in American history, having also been used during the Civil War. Both the Union and the Confederacy resorted to conscription laws, like the Enrollment Act, as voluntarism waned and the need for manpower became critical.
The country's first peacetime draft was established in September 1940, although the initial draftees were to serve for only one year. With the attack on Pearl Harbor and the United States officially joining the war, a massive draft program was initiated to meet the predicted need for around nine million servicemen. Over the course of World War II, about fifty million American men registered for the draft, and ten million were inducted into service.
Thus, the correct option for how the United States met the demand for soldiers once America entered the war is d. Mandatory conscription.