Answer:
This would most likely be the Middle Passage, and therefore C.
Step-by-step explanation:
Let's go each answer one by one.
First, the columbian exchange was more of trade between the newer North America (New World) and Europe (Old World). This included things like livestock, food, culture, and diseases. While slaves were traded, they were not done in such quantities as 12 million.
Next, the Triangular Trade. While the Triangular Trade did include a cycle of trade of raw materials to Europe. finished goods to Africa, and manpower to America, the requirements asked for may be applicable for this event, but another choice is more specific and therefore better. (Even then, the Triangular Trade has Europe, America, and Africa. Not just the African x America.)
Then we have the mercantilist theory. Maximise exports, minimize imports. Has near nothing to do with the requirements on a foundational level.
That leaves us with C. The Middle passage was a portion of the Triangular Trade, but it was only between Africa & America. While roughly 12 million slaves were transported, it took around 80 days with a 15% death rate which might mean those requirements might be taken from a different source. With that in mind however, it can be seen that C is the best option.