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%.