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

a) fertility, mortality, and migration

b) mobility, immigration, and population density

c) demography, climate, and climate

d) population density, demography, and landforms

A : Fertility, mortality and migration.

Explanation:

The rate of total population is reflected in these three items. Fertility  (births) are measured by how many babies are born during the day, month or year. Mortality is the ratio of deceased people measured by the same calculations as Fertility.

Migration is the physical movement of people from one place to another. It can be between countries and it can be within the country itself.

The total population is a computation of the fertility, mortality and migration in total, over one particular period of time chosen to be calculated.