How did the United States mobilize 12 million military service members by 1945?
A) by using a combination of required registration for the draft and appealing to Americans' patriotic duty to volunteer
B) by conscripting all American men between the ages of 18 and 45 to serve in me Army, Navy, or Marines
C) by using financial incentives to encourage young men to volunteer in exchange for generous bonuses after the war
D) by allowing women to serve in all military roles to encourage greater overall enlistment numbers

Adopting policies like Cost-Plus Contracts to encourage firms to transition to war manufacturing in order to manufacture materials and vehicles as rapidly as feasible as part of the US mobilization for WWII.

The Selective Service and Training Act (draft), as well as soldier training and deployment, were all part of the US mobilization for WWII.

Option A is correct as by combining draft registration requirements with an appeal to Americans' patriotic responsibility to volunteer the US mobilizes 12 million military service members.

The other Options are incorrect as:

Option B is incorrect as no US did not mobilize men between the age of 18 and 45 to serve the country.

Option C is incorrect as they did not use financial incentives to encourage young men.

Option D is incorrect as they did not allow women to serve in military roles.

Thus Option A is correct as this was the way the United States mobilize 12 million military service members by 1945.

For more information about the United States mobilizing 12 million militaries:

https://brainly.com/question/24024595