Final answer:
Antigenic drift (option c) refers to small changes in the influenza virus due to mutations, leading to milder epidemics, while antigenic shift causes more significant changes, potentially leading to pandemics.
Step-by-step explanation:
The evolution of the influenza virus that causes relatively mild and limited disease epidemics is referred to as antigenic drift. Antigenic drift is characterized by point mutations in the genes for the surface proteins neuraminidase and/or hemagglutinin, which result in small antigenic changes over time. This process leads to the gradual evolution of the virus, allowing it to evade the immune system to some extent, but generally does not lead to widespread pandemics.
In contrast, antigenic shift refers to a more dramatic change that occurs when two different influenza viruses infect the same host cell and engage in gene reassortment, resulting in a new virus with a mixture of surface proteins from the original viruses. This can lead to the development of a significantly different influenza virus against which people have little to no immunity, potentially causing a pandemic.