Which of the following medications is an antibiotic?

Acyclovir is an antiviral medication used primarily to treat infections caused by certain types of viruses, including herpes simplex virus and varicella-zoster virus. Unlike antibiotics, which target bacteria, antivirals are designed to inhibit viral replication and are ineffective against bacterial infections.

Oxytocin is a hormone that plays roles in childbirth and lactation. It is not classified as an antibiotic and is primarily used in clinical settings to induce labor or control bleeding after childbirth. Its mechanism of action is entirely different from that of antibiotics, focusing on physiological processes rather than bacterial eradication.

Phenazopyridine is a urinary analgesic that relieves pain, burning, and discomfort caused by urinary tract infections and other urinary conditions. However, it does not possess any antibacterial properties and is not used to treat the infections themselves; instead, it addresses the symptoms associated with them.