Final answer:
The spread of diseases was the largest cause of human migration during the Columbian Exchange, as European diseases decimated up to 90% of the Native American population, having a more profound impact on human movement than trade, religious efforts, or the search for new land.
Step-by-step explanation:
The largest cause of human migration during the Columbian Exchange was c) Spread of diseases. The arrival of Europeans in the Americas brought with them diseases to which the indigenous populations had no immunity. This led to widespread epidemics that decimated the Native American population, which in some estimates resulted in the death of up to 90 percent of the indigenous people.
In the context of the Columbian Exchange, while there were certainly motivations related to trade for valuable goods, religious conversion efforts, and the search for new land resources, none of these matched the impact that the spread of diseases had on human migration and mortality. The introduction of diseases like smallpox and influenza had a catastrophic effect, causing massive upheaval and reconfiguring of entire societies in the Americas.