Final answer:
None of the options provided accurately represents the fraction of the planet's population that was Native American when the Vikings first arrived in America, with estimates suggesting Native Americans were roughly 1/5 to 1/6 of the world population.
Step-by-step explanation:
When the Vikings first arrived in America, estimates suggest that there were approximately seventy-five million Native Americans compared to Europe's population of about seventy million. It is difficult to provide an exact fraction of the planet's population that was Native American during the Viking era, primarily because global population figures from the time are uncertain.
However, with these rough numbers in mind and given that the world population at the time was likely between 360 to 450 million, it can be inferred that Native Americans were roughly 1/5 to 1/6 of the world population. This fraction is not among the options provided in the question, and based on the estimates none of the options offered (a) 1/3, (b) 2/3, (c) 1/2, (d) 1/4) accurately represents the fraction of the planet's population that was Native American when the Vikings arrived.