Name an antiviral that is effective against both HIV and Hepatitis B.

Lamivudine is an antiviral medication that is effective against both HIV (Human Immunodeficiency Virus) and Hepatitis B virus (HBV). It functions as a nucleoside reverse transcriptase inhibitor (NRTI), which means it interferes with the reverse transcriptase enzyme that is critical for the replication of these viruses. By inhibiting this enzyme, lamivudine effectively reduces the viral load in patients suffering from both HIV and HBV, making it a versatile option in treating these infections.

In contrast, the other options listed do not have overlapping efficacy. Daclatasvir is primarily used for the treatment of Hepatitis C and does not act on HIV. Ribavirin, while effective for Hepatitis C, does not have significant effects on HIV. Oseltamivir is an antiviral used specifically to treat influenza and has no activity against either HIV or Hepatitis B. Therefore, lamivudine stands out as the suitable choice for acting against both viruses.