Explanation:

- Option 1: Carrying capacity

This refers to the maximum population size that an environment can support indefinitely, without being degraded. It’s more about environmental limits than about people moving for better opportunities.

- Option 2: Brain drain

This is the correct term. It describes the emigration of a country’s highly skilled, highly educated professionals to other nations in search of better economic and social opportunities.

- Option 3: Demographic transition

This is a model that describes population change over time, typically associated with transitions in birth and death rates as a society develops.

- Option 4: Closed population

This occurs when a population is not affected by inward or outward migration. It grows or declines based only on births and deaths.