The main difference between the Pan American Games and the Olympic Games among the provided responses is:
Only countries in the Americas and Caribbean participate in the Pan American Games, while the Olympic Games are worldwide.
This accurately highlights the geographical scope of participation in each event, contrasting the regional focus of the Pan American Games with the global participation in the Olympics.
The other responses contain inaccuracies:
- Professional athletes have competed in both events, especially after changes to the rules.
- The Pan American Games are not limited to Buenos Aires; they occur in various cities throughout the Americas.
- The Pan American Games are held every four years, similar to the Olympics, not every six years.