I think the answer is that The 1850s was a pivotal decade in the 19th century. In the United States, tensions over slavery became prominent and events began to put the nation on the road to civil war. In Europe, new technology was celebrated and the great powers fought the Crimean War.
The triangle stopped first at the the West Indies from Africa where most slaves were pulled off board, then sent on to ports such as New Orleans to be sold. In this context, the closest would be A.
