The correct answer is A) European diseases wiped out much of the indigenous labor force.
The Europeans look to Africa for a source of labor to use in the Americas because European diseases wiped out much of the indigenous labor force.
The Europeans had contagious diseases such as smallpox and yellow fever, that affected the indigenous population in the Americas. African people had been exposed to those diseases before, just like the Europeans, so the Africans had similar immunity and they were more resistant to those diseases than the indiginous of the Americas. For instance, African people developed better resistance to Malaria.