Which of the following is used to treat frounce?

Metronidazole (Flagyl) is the appropriate treatment for frounce, which is a disease caused by the protozoan parasite Trichomonas gallinae, primarily affecting birds of prey. This medication is effective because it targets anaerobic bacteria and certain protozoa, making it suitable for treating infections caused by Trichomonas.

Other options, such as ibuprofen and aspirin, are analgesics and anti-inflammatory medications that do not possess antibacterial or antiparasitic properties effective against fouce. Amoxicillin, while an antibiotic, is not effective against protozoan infections and is primarily used for bacterial infections. Therefore, metronidazole is the only option that directly addresses the cause of frounce, making it the correct choice for treatment.