Final answer:
The sentence 'I fear you have said something you will regret' showcases a formal tone by using respectful language without informal expressions or idioms.
Step-by-step explanation:
A “formal tone” is often used in research papers and reports, and typically includes writing objectively, using precise language and specifics, and avoiding conversational phrasing. The sentence that best reflects a formal tone is: I fear you have said something you will regret. When opting for a formal tone in writing or speech, it is pertinent to use proper grammar and choose words that are respectful and devoid of informal or slang expressions.
This particular sentence avoids colloquial language and presents the concern in a serious and courteous manner, characteristic of a formal tone. In comparison, the other options contain casual language (like 'babbling' and 'talkin') or idiomatic expressions ('stuck your foot in your mouth'), which are generally not suited for formal communication.