Correct Answer:
Option A. Influenza
During 1918 - 1919, an influenza epidemic killed about more than 20 million people. This epidemic killed more people than were killed due to the World War 1 and is said to be the most devastating epidemic of the history. Among the US troops which were deployed in Europe, more than half of deaths were a result of influenza rather than the killings done by enemy.
Oh well then It'd be conquest or in other words, force
A it led to conflicts with the Republican Congress on ape.x