Estimates suggest that if the wars of the 20th century had killed the same proportion of the total population as ancient tribal wars did, then the death toll from those conflicts would have exceeded people.

2 billion people

Explanation:

The attrition rate of the ancient tribal wars was really brutal and produced casualty rate of almost 60% of the population at the time. If the proportion which is 60% is likened to the 20th century wars and 60 percent of people were killed, this population will exceed two billion because the 20th century recorded population of up to 4 billion people and if 60 percent are wiped, then this figure will exceed 2 billion people.

the right answer is a