To start, the equation needed to represent this problem would be an equation: y= ab^x
a represents the initial amount. In this case, the initial amount would be 4 billion. It would be represented as 4 since each 1= one billion.
b represents the rate. Because the problem discusses a growth, convert the rate, 1.9% to a decimal and add 1 to it (add one if it is a growth, subtract one if it is a decay). b would be 1.019.
x represents the time in which the population grows or t.
When you piece this together, your equation should look like: y=4(1.019)^t
or
Choice A.
Hope this helps and have a nice day! Feel free to ask any questions about my work. :)