Final answer:
Tobias, a character studying plans for the 1893 Chicago World's Fair, is probably fictional, whereas the World's Fair itself and the exhibition celebrating Columbus are historical facts.
Step-by-step explanation:
The part of the story that is probably completely fictional is 'Tobias studying plans for the fair.' The Chicago World's Fair, also known as the World's Columbian Exposition, did indeed take place in 1893 and celebrated the four-hundredth anniversary of Columbus's arrival in the New World. However, Tobias as a character is likely a fictional creation, and his personal actions, such as studying plans for the exhibition, would be a fictional element within the context of the historical setting.