Which of the following is most likely to be found in a dietary supplement?

Caffeine is commonly found in dietary supplements, particularly those targeted towards athletes or individuals looking to enhance their physical performance or energy levels. It is well-known for its stimulant effects, which can improve alertness, endurance, and focus during exercise. Because of its widespread use and popularity, many supplement manufacturers incorporate caffeine into their products to attract consumers seeking an energy boost.

The other options, while they have their own benefits in specific contexts, are not typically included in dietary supplements. Prescription medications require a doctor's approval and are not available over-the-counter in the same way dietary supplements are. Antibiotics are restricted to treating bacterial infections and are not suitable for general supplementation. Analgesics, which are used for pain relief, may be found in over-the-counter medications but are not common in dietary supplements. Therefore, caffeine stands out as the most likely ingredient to be found in a dietary supplement due to its performance-enhancing and energizing properties.