A"brain drain" is:
a. when a job zaps people’s energy so they perform poorly.
b. highly educated people in poor countries migrate to wealthier countries to increase their return on their physical capital investment.
c. poorly educated people in poor countries migrate to wealthier countries to increase their return on their human capital investment.
d. highly educated people in poor countries migrate to wealthier countries to increase their return on their human capital investment.

Brain drain refers to Option D. Highly educated people in poor countries migrate to wealthier countries to increase their return on their human capital investment.

Explanation:

Brain drain occurs when a person who is smart and talented in a field leaves their home country and then emigrates to take up work and to lend their talents to the social system and industry and educational system in another country. This is a dilemma for many poor nations because they cannot retain their talented and highest performing citizens who have good incentives and reasons for wanting to move abroad where they can earn a higher income and advance their research to a greater extent in the case of hard sciences for example.