Name the antiviral agent that is an interferon.

The antiviral agent that is classified as an interferon is Peginterferon alfa. Interferons are a group of signaling proteins that are produced and released by host cells in response to the presence of pathogens such as viruses and bacteria, as well as tumor cells.

Peginterferon alfa is specifically used in the treatment of viral hepatitis and certain types of cancer. It operates by enhancing the immune response against the viruses and can inhibit viral replication, making it effective in treating conditions like hepatitis B and C. The "peg" in Peginterferon indicates that it has been chemically modified for a longer half-life, allowing for less frequent dosing compared to non-pegylated interferons.

While Peginterferon beta, Interferon gamma, and Interferon epsilon also serve various roles in immune response and antiviral activity, their specific indications and mechanisms differ. Peginterferon beta is primarily used for multiple sclerosis. Interferon gamma is predominantly involved in enhancing immune responses and is not primarily classified as an antiviral. Interferon epsilon, though less common, does not have established therapeutic uses as the other forms do. Thus, Peginterferon alfa stands out as the correct answer in this context.