Final answer:
The Selective Service Act of 1917 required all men aged twenty-one through thirty, and later eighteen to forty-five, to register to the draft, via civilian draft boards, for potential military service in WWI. The correct answer is option B.
Step-by-step explanation:
The Selective Service Act of 1917 mandated that all men of a certain age had to register for the draft, but not necessarily serve in combat unless drafted. Specifically, it required all men aged twenty-one through thirty to register, and then in 1918, the age range was expanded to include all men between eighteen and forty-five. The draft was instrumental in building the American military forces during World War I, eventually resulting in over 2.8 million men serving in the American Expeditionary Force.
Local civilian draft boards managed the registration process, which sometimes resulted in uneven enforcement and varied interpretations of service qualifications and deferments. In practice, this system created a pool of potential soldiers who could be called up for military service as needed, rather than requiring immediate and universal active duty.