What would you say to someone who thinks that America was discovered by the Vikings, and not by Columbus?

I would say he is right because the Vikings were the first discoverers of America. At that time, for most of the civilized world, this discovery was inaccessible and unknown. But when Columbus made such a discovery, many European states learned about it. In addition, most of them had large ships in order to make long voyages. This is how the era of the great geographical discoveries began.