Final answer:
The first thing that immigrants at Ellis Island encountered was a physical examination by a medical professional to prevent the introduction of contagious diseases to the country. Therefore, the correct option is B.
Step-by-step explanation:
The first thing that immigrants at Ellis Island encountered was physical examination by a medical professional. Upon arrival, doctors or nurses inspected the immigrants for any signs of infectious diseases. This examination was aimed at preventing the introduction of contagious diseases to the country.