The correct answer is Aspirin.

- Reye's syndrome is a rare but serious condition that causes swelling in the liver and brain, primarily affecting children and teenagers recovering from viral infections such as influenza or chickenpox.
- One of the key risk factors identified for developing Reye's syndrome is the use of aspirin (acetylsalicylic acid) during viral illnesses.
- Because of this association, the use of aspirin in children and adolescents with viral infections is strongly discouraged.

- Ibuprofen and paracetamol (acetaminophen) are generally considered safe alternatives for managing fever and pain in children with viral infections.
- Diphenhydramine is an antihistamine and does not have any known association with Reye's syndrome.
- The exact cause of Reye's syndrome is not completely understood, but it is well established that aspirin use during viral illnesses significantly increases the risk.

Therefore, to minimize the risk of Reye's syndrome, aspirin should be avoided in children and teenagers recovering from viral infections.

Reference: Harrison's Principles of Internal Medicine, 20th Edition, Volume 1, Chapter 338: Reye's Syndrome