The sentence that best reflects a theme in "The Piece of String" by Guy de Maupassant is:
Guilt can consume a person's perspective.
In the story, the protagonist, M. Hauchecorne, becomes embroiled in suspicion and guilt over a seemingly trivial matter, which ultimately distorts his reputation and affects how he is perceived by others. The overwhelming nature of guilt and its impact on one's life and perspective is central to the narrative.