What substance is prohibited in racehorses to ensure fair racing?

The correct choice, which identifies performance-enhancing drugs as prohibited in racehorses, underscores the commitment to maintaining the integrity of horse racing. Performance-enhancing drugs can artificially boost a horse's speed, strength, or endurance, providing an unfair advantage over competitors. This practice undermines the essence of competition, where outcomes should reflect the natural abilities of the horses and the training they receive. Regulatory bodies like the Texas Racing Commission implement strict policies against the use of these substances to ensure a level playing field and protect the welfare of the horses involved in racing.

In contrast, vitamins, minerals, and natural supplements are generally allowed when used within appropriate guidelines, as they are intended to support a horse's health and well-being rather than enhance its performance in a way that compromises fairness in racing.