The correct answer is:
The US.
During WWI, the US had a total of 117,466 deaths, 0.13% of its entire population.
The UK, including its colonies, had at least 867,829 deaths, 1.91% of the total population.
Italy had 1,052,400 deaths, a rate of 2.96%.
Serbia had at least 750,000 deaths, a rate of 16.67%.
