Final answer:
Ivan van Sertima's claim about Europeans in Mesoamerica before Columbus is controversial and not widely accepted. The statements about the slaughter of natives, the importance of Indian alliances, and the role of native allies in the Aztec conquest are true. Bartholomew de las Casas was a critic of Europeans' treatment of indigenous peoples.
Step-by-step explanation:
Ivan van Sertima's argument that Europeans came to Mesoamerica before Columbus is a topic of debate among historians. While there is evidence of Norse exploration in North America, specifically Newfoundland, several centuries prior to Columbus's voyage, the claim that Europeans other than the Norse visited Mesoamerica before Columbus is not widely accepted and is considered controversial among scholars. It is important to note that much of the evidence presented by van Sertima has been critiqued for lacking sufficient archaeological proof.
The majority of the natives killed during the exploration period were indeed slaughtered by Europeans who possessed superior weapons, making statement A true. Furthermore, Indian alliances were crucial for establishing power in the southeast for the European empires, validating that A is also true. The colonizationist scheme in the early 1800s had diverse views among black abolitionists, and while some supported it, the statement that it was 'popular' might require more nuanced exploration. Finally, recruiting native allies was significant in the Spanish conquest of the Aztec, making statement A true.
Bartholomew de las Casas was notably vocal in his criticism of the Europeans' treatment of the Indians of Meso- and South America, making answer C correct. Columbus's own perceptions and misjudgements reflect the common European bias of the time, related to the belief in European superiority and the preconceived notion of natives being subordinate.