Explanation:

- Option 1: Depopulation

- This refers to a decline in the population of a particular area. It can happen due to reasons like high mortality, migration, or low birth rates.

- Option 2: Migration

- Migration is the movement of people from one place to another, across regions or territories. It can be within a country or across international borders for reasons such as work, education, or safety.

- Option 3: Mortality

- Mortality is the rate of death in a population. It helps in understanding the health and longevity of a population.

- Option 4: Demographic Transition

- This is a theory that describes the shift from high birth and death rates to low birth and death rates as a society progresses economically and socially.