The african replacement model (arguing that modern homo sapiens first appeared in one place, and then spread out to other parts of the world) suggests that modern homo sapiens:
a. evolved in africa and migrated into the rest of the world replacing all other populations
b. evolved in africa and migrated directly in to americas
c. evolved in europe and migrated to africa
d. evolved locally in europe, africa, and asia from populations of archaic humans

A. Evolved in Africa and migrated into the rest of the world replacing all other populations.

Explanation:

The origin of human races has remained as a much debated point among the scientists studying the human evolution. Many scientists believe that different races evolved independently from Homo erectus and they adapted in into different places like Orientals in Asia, Caucasians in Europe, Aborigines in Australia and so on. But it is highly unlikely to believe that same species evolved more than once in different places.

Other scientists believe that human races appeared after Homo sapiens evolved from Homo erectus. The mitochondrial DNA study from human beings all over the world clearly shows that all human races originated from one Homo sapiens ancestor in Africa.  Since the mitochondrial DNA is transmitted only by the females, scientists looked at it to study evolution. So far it has been observed that greatest number of different mitochondrial DNA sequences occur among the Africans and since the DNA accumulates mutations over time, the oldest DNA should show the largest number of mutations. These finding suggest that humans have been living in Africa longer than on any other continent. This supports the African replacement model which argues that H. sapiens evolved in Africa and human races evolved after that.

as resources become scarce because of population growth, the growth decreases after few years due to lack of resources and food.