Demographic transition theory can be used to describe and predict the future population of any area. The theory tells us that population of any region changes from high births and high deaths to low births and low deaths as society progresses from rural agrarian and illiterate to urban industrial and literate society. These changes occur in stages which are collectively known as the demographic cycle.

Which of the following is not a natural factor which causes the population of a region to change?

Migration

The correct answer is Option 3: Migration

Birth rate and death rate are considered natural factors affecting population change because they occur biologically within a population. Migration, on the other hand, refers to the movement of people from one place to another and is treated as a social or human factor rather than a natural factor. Hence, Option 3 is correct.