Answer:
D. Mobilization
Step-by-step explanation:
Mobilization is the process by which an authority quickly organizes troops in order to bring them to a state of readiness for conflict. So, it was this process that allowed countries to call up troops and move them quickly.
The other options are incorrect because conscription refers to the involuntary military service that an authority demand to certain people, patriotism is the idea of supporting one's country, people and culture, and militarism refers to an ideology that sees the military as the most important aspect of society.