Final answer:
The best excerpt establishing the setting in 'A Piece of String' is 'It was market day in Goderville,' as it informs about the time and place.
Step-by-step explanation:
The excerpt that best establishes the setting in Guy de Maupassant's short story A Piece of String is 'It was market day in Goderville.' This line provides a direct indication of both the time and the place where the events are unfolding, giving the reader a specific context and background against which the story takes place. It sets the stage for the encounters and the story's plot to develop, as opposed to a general description of the environment or a character's action.