C is my best guess because A and D are wrong.
Because:
Europeans were interacting with a big chunk of the old world - Europe, Middle East, Central Asia, East Asia, South Asia, North Africa. There were a lot of migrations, invasions and over 3 millennia of constant interaction. This is likely to have gotten them immune to a lot of diseases.The population density of the new world was pretty low and native American populations were isolated from each other. This means many diseases might not have had the critical mass to be a big part of the new world before the travelers came and spread things all over.