The sentence that best reflects a theme in "The Piece of String" by Guy de Maupassant is:
People's opinions sometimes matter more than the truth.
In the story, the main character, Maitre Hauchecorne, is wrongly accused of theft, and despite his attempts to prove his innocence, the townspeople's opinions lead to his social downfall. This illustrates the theme of how public perception can overshadow the actual truth.