Answer:
Sub-developed countries.
Step-by-step explanation:
According to statista.com, countries with the highest population growth are South Sudan, Angola, Malawi, Burundi, Uganda, Niger, Mali, Burkina Faso, Zambia and so on.
The common characteristic these high-population-growth countries have is sub-development. They all are sub-developed countries, which implies several variables: Low income per capital, Lacking Human Resources, Vulnerable Economy.
All these variables, derived from sub-development are a common point in these countries, which can allow us to deduct that probably this is the reason.