As countries get richer their population growth rate tends to

B. grow exponentially

Explanation:

When countries become richer, services aimed at the population and offered by governments have more quality, which provides a high quality of life for the population.

When a country gets rich, the unemployment rate decreases, health services are faster and more efficient, the country's economy improves, people get better housing, basic sanitation, education and many other improvements. As a result, people tend to have more children, and these children are better able to survive until they have their own children. As a result, the population growth rate grows.

We can say that this is an exponential growth, since as the population grows from the growth of the country's wealth.

age range would be the answer