Which drug is an antiviral medication used for shingles treatment?

Acyclovir is the correct choice as it is an antiviral medication specifically indicated for the treatment of shingles, which is caused by the varicella-zoster virus. This medication works by inhibiting the viral DNA synthesis, helping to reduce the severity and duration of the shingles outbreak.

When considering the other options, they serve entirely different purposes. Ibuprofen is a non-steroidal anti-inflammatory drug (NSAID) used to relieve pain and reduce inflammation but does not treat the viral infection itself. Amoxicillin is an antibiotic, effective against bacterial infections, and does not have any antiviral properties, making it ineffective for shingles. Prednisone is a corticosteroid that may be used to reduce inflammation but again does not directly combat viral infections. Therefore, Acyclovir stands out as the specific antiviral treatment indicated for shingles.