Question

Columbus explored all of the following except: South America North America Caribbean Islands Central America

Answer 1

Although it's a bit counter-intuitive, Columbia actually never explored North America, instead visiting the other locations on this list instead. This lead to great discoveries but lots of suffering for the local inhabitants. 
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Answer 2

Columbus never explored North America.

But, here is who did explore North America: Several theoretical contacts have been proposed, but the earliest physical evidence comes from the Norse or Vikings. Erik the Red founded a colony on Greenland in 985 CE. Erik's son Leif Eriksson is believed to have reached the Island of Newfoundland circa 1000, naming the discovery Vinland.