Answer: d) Internal migration.
Step-by-step explanation:
Mexico City had been the ultimate goal for Mexican internal migration, mainly because of the economic opportunities and the developed infrastructure in education, culture, and health. The 1960s saw the greatest increase in the city´s population in its entire history, still, mostly because of in-migration. Nevertheless, the city’s immigration eventually decreased, getting a negative net migration between 1985 and 1990.