Final answer:
The best indicator that a person is an authority on a subject is having education and experience in that subject.
Step-by-step explanation:
The best indicator that a person is an authority on a subject is option D) The person has education and experience in the subject. Being knowledgeable and experienced in a subject is a strong indication that someone has specialized expertise and can be considered an authority in that field. Education provides a foundational knowledge base, while experience allows for real-world application and further mastery of the subject. This combination enables individuals to understand the intricacies of the subject, provide accurate information, and make informed decisions.