Which is an accurate description of the columbian exchange?
a. the migration to the americas of europeans seeking wealth and opportunity
b. the movement of native americans to europe after columbus's voyage
c. the disastrous introduction of unknown species to european ecosystems
d. the transfer of plants, animals, people, and diseases between hemispheres

The Columbian Exchange was the arrival European people, plants, animals, and diseases in the Americas.  It happened when Columbus and subsequent explorers arrived in North and South America.

So, which of these options is the most similar?  It's not A, the migration to the Americas of Europeans seeking wealth and opportunity.  That came much later.  B, the movement of Native Americans to Europe after Columbus's voyage is also incorrect: they had no wish to move from their native lands.  C, the disastrous introduction of unknown species to European ecosystems, isn't right either.  Not many new species reached Europe, and they had little effect there.  The transfer of plants, animals, people, and diseases between hemispheres, D, however, is correct!  (Keep in mind that the Americas were in one hemisphere, and Europe is in the other.)

D