Which of the following substances can be defined as a drug?

Ibuprofen is defined as a drug because it is a medicinal substance primarily used to relieve pain, reduce inflammation, and lower fever. It belongs to the non-steroidal anti-inflammatory drugs (NSAIDs) category, which are well-recognized in pharmacology for their therapeutic effects on various conditions such as headaches, arthritis, and muscle pain.

In the context of the other substances, while they too can be classified as drugs due to their therapeutic effects—lisinopril being an antihypertensive, aspirin used for pain relief and anti-inflammatory purposes, and caffeine often consumed for its stimulant effects—the correct answer focuses on a more common over-the-counter medication known specifically for its pain-relieving applications. Each of these options is indeed a drug; however, the emphasis on ibuprofen may be derived from its widespread use and recognition in both pain management and inflammation.