The correct answer to this open question is the following.
Although there are no options attached we can say the following.
What caused the sickness among the slaves that Equiano describes in this passage is that these slaves traveled in inhuman conditions because they were put in overcrowded spaces with no air or ventilation at all. This caused most of them to get sick and diseases spread easily.
Equiano’s account is evidence of the poor conditions under which enslaved people were transported and then sold.
This reflects the way slaves were treated during those horrible years of slavery and the Slave Triangle that traded African slaves to the Americas. In 1789, Olaudah Equiano wrote the book "The Interesting Narrative of the Life of Olaudah Equiano, or Gustavus Vassa, the African," which describes his memories of being treated as a slave since he was 11 years old.
Olaudah Equiano's narrative shows how the interaction among the peoples of Africa and Europe impacted the course of slavery.