Final answer:
The detail that best contributes to an informal tone in this excerpt is the use of the word 'whispers'.
Step-by-step explanation:
The detail that best contributes to an informal tone in this excerpt is the use of the word 'whispers'. In an informal tone, the author uses casual and conversational language to create a friendly and relaxed atmosphere.
The use of 'whispers' suggests a more personal and intimate connection with the subject matter, rather than a formal and distant tone.