Final answer:
The best example of a primary source is a copy of an interview with the team's first coach, as it is an original document created during the time under study.
Step-by-step explanation:
The best example of a primary source from the provided options is a copy of an interview with the team's first coach. A primary source is an original document or record that was created at the time under study. These sources are direct evidence of the subject being researched and have not been filtered through interpretation or evaluation by a second party. Options like a book that summarizes the team's history or a magazine article offering opinions about the team's early leaders are considered secondary sources because they interpret and analyze primary sources. Meanwhile, a highlight video showing the team's accomplishments, although closer to the events, is still a compilation and not an original record like an interview. Therefore, the interview with the team's first coach is the clearest example of a primary source as it likely contains firsthand insights and information about the team's formation and early history.