The primary reason the Portuguese turned to enslaved Africans to work on sugar plantations was that Indigenous Americans in the Caribbean were nearly exterminated by disease and Europeans did not want to work in such brutal conditions. The indigenous populations suffered catastrophic declines due to diseases brought by Europeans, such as smallpox, as well as forced labor and violence.
As the demand for sugar grew and labor became scarce, the Portuguese and later other European powers began to import enslaved Africans. Africans were resilient to tropical diseases and had agricultural knowledge that made them suitable for plantation work. Thus, the combination of a depleted indigenous workforce and the unwillingness of Europeans to work in harsh conditions led to the reliance on African slaves for labor on sugar plantations.
So, while the statement "Indigenous Americans in the Caribbean were nearly exterminated by disease and Europeans did not want to work in such brutal conditions" captures the key factors in this historical context, the other statements provided are either inaccuracies or not the primary reasons for the shift to African slavery.