The factor that best describes why many Africans were enslaved as part of the Triangular Trade is: The work needed for large plantations required much larger volumes of enslaved peoples.
The demand for labor on plantations in the Americas, particularly for cash crops like sugar, tobacco, and cotton, drove the need for a large workforce, which led to the enslavement of millions of Africans. While other factors played a role in the broader context of the Triangular Trade, the primary reason for the enslavement of Africans was the demand for agricultural labor on plantations.