Answer:
A. introduction of the horse to the New World
Step-by-step explanation:
The positive effect of the Columbian exchange is the "introduction of horses to the New World."
Unlike the other available options, the introduction of horses to America ensures the American Indians are exposed to horses which have so many benefits to the people at the time, including a means of transportation. It is later used together with Cart to improve the transportation system in the New World.
The options are adverse effects of the Columbia exchange.