Which of the following types of viruses is MOST likely to undergo an abrupt major antigenic shift, permitting reinfection in a previously exposed individual?

The phrase "antigenic shift" should tip you off to look for "influenza viruses" or "ohomyxoviruses" in the answers. The ohomyxoviruses include influenza viruses A, B, and C. These viruses are successful at reinfecting previously exposed individuals because their genome consists of about eight (the viral packaging process is a little vague and sometimes entraps more pieces) separate segments of RNA. Minor antigenic changes are frequent and are called antigenic drift. Additionally, major changes, called antigenic shift rather than antigenic drift, rarely occur, possibly as a result of double infection of cells by human and animal influenza viruses with resulting accidental exchange of whole RNA segments. The other viruses listed in the answers do not undergo such dramatic shifts.