Which of the following medications is an interferon beta-1a?

Rebif is indeed an interferon beta-1a medication, which is primarily used to treat multiple sclerosis (MS). It functions by modulating the immune system to help reduce the frequency and severity of MS relapses. Interferons, including beta-1a, work by interfering with viral replication and the ability of immune cells to attack the central nervous system, which is significant in the pathophysiology of MS.

The other options listed do not classify as interferon beta-1a. Gilenya is designed for MS but is a sphingosine-1-phosphate receptor modulator, not an interferon. Bimatoprost is primarily used to treat glaucoma and promote eyelash growth, having no relevance to MS treatment. Amiodarone is an antiarrhythmic medication used in treating and preventing certain types of serious irregular heartbeats, not related to the interferon category. Thus, the identification of Rebif as the correct answer underscores its role and mechanism within MS therapy.