The sentence that best reflects a theme in "The Piece of String" by Guy de Maupassant is:
Truth does not always lead to justice.
In the story, the protagonist, Maitre Hauchecorne, is falsely accused of theft, and despite his insistence on his innocence and the truth of his actions, he faces social ostracism and punishment. This highlights the theme that truth does not guarantee justice or fairness in society.