What is considered prima facie evidence of prohibited substance administration in a horse?

The correct answer is based on the significance of laboratory findings in establishing the presence of prohibited substances in horses. A finding by a Commission-approved laboratory serves as prima facie evidence because it provides scientifically validated results demonstrating that a banned substance is present in the horse’s system. This type of evidence is significant in regulatory contexts, as it offers an objective basis for determining whether a horse has been administered prohibited substances, which is essential for maintaining fair competition and animal welfare in horse racing.

In contrast, other choices do not provide the same level of reliable evidence. For instance, a horse's winning status or finishing position does not inherently indicate any substance influence, as many factors can affect race outcomes. Similarly, while a trainer's admission may be a compelling factor, it can be subjective and less reliable compared to scientific evidence. Therefore, a finding from a Commission-approved laboratory stands out as clear and conclusive proof in matters regarding substance administration.