Final answer:
The informal tone is established by the folklore-esque personification of whispers from the ancestors and the casual storytelling manner in which the president's family history is described.
Step-by-step explanation:
The detail that best contributes to an informal tone in the excerpt is the personification of whispers from the ancestors, suggesting a folklore-like quality and a colloquialism combining the mystic with the everyday.
This tone contrasts with the more formal, objective descriptions in the other presented excerpts, which discuss family history and ancestral pride without the same level of whimsical or conversational language. The use of imaginative language and the invocation of 'powerful spirits' rather than simply stating the historical significance of the family gives the passage an informal, story-telling nature.