To find the increase in the world's population from 1500 CE to 2020, we subtract the population in 1500 from the population in 2020:
\[ \text{Population in 2020} - \text{Population in 1500} = 7.8 \times 10^9 - 4.5 \times 10^8 \]
First, we need to express both numbers in the same format. We can rewrite \(4.5 \times 10^8\) in terms of \(10^9\):
\[ 4.5 \times 10^8 = 0.45 \times 10^9 \]
Now we can subtract:
\[ 7.8 \times 10^9 - 0.45 \times 10^9 = (7.8 - 0.45) \times 10^9 = 7.35 \times 10^9 \]
Thus, the increase in the world's population is:
\[ \boxed{7.35 \times 10^9} \]
The correct answer is D. 7.35×10^9.