Which medication is an antiviral used to treat influenza?

Oseltamivir is the correct answer because it is an antiviral medication specifically indicated for the treatment of influenza. It works by inhibiting the enzyme neuraminidase, which is essential for the replication and spread of the influenza virus within the respiratory tract. This action helps to shorten the duration of flu symptoms and may reduce the risk of complications associated with the illness when administered early in the course of infection.

In contrast, the other medications listed serve different purposes. Azithromycin is an antibiotic used to treat bacterial infections, not viral infections like influenza. Omeprazole is a proton pump inhibitor that reduces stomach acid, primarily used for treating conditions such as gastroesophageal reflux disease (GERD). Metformin is an oral hypoglycemic agent used to manage type 2 diabetes by reducing glucose production in the liver and improving insulin sensitivity. These differences in mechanism and indications clarify why oseltamivir is the appropriate choice for treating influenza.