The total population of an area results from the interaction of two forces. The two forces are

1) migration and natural increase

2) migration and immigration

3) immigration and natural increase

4) emigration in natural increase

The two forces that interact to result in the total population of an area are migration and natural increase.

Explanation:

Natural increase in the population has been an inevitable force, for the birth rate anywhere in the world is higher than the death rate. On the other hand, the term 'migration' encompasses both 'emigration' and 'immigration'. As both emigration and immigration are crucial factors in the determination of the population of an area, the term migration is most suitable to be called as the force that interacts with the force of natural increase to result in the population of an area.

1. That is the correct answer.