Which term means to prevent from being successful or to thwart?

The term that means to prevent from being successful or to thwart is "foil." In a more detailed context, "foil" originates from the practice of covering a sword with a thin layer of foil to prevent it from being used effectively, thus symbolically representing the act of thwarting an attempt or strategy. In literature and conversation, to foil someone’s plans typically implies intervening or obstructing their goals, thereby ensuring their failure in a given situation.

The other terms do not convey the same meaning. "Forfeit" generally refers to losing or giving up a right or privilege, often as a penalty. "Forgery" pertains to the act of falsifying documents or artworks, and does not involve thwarting someone’s efforts. "Flaunt," on the other hand, means to show off proudly or ostentatiously, which contrasts entirely with the idea of hindering or preventing success. Therefore, "foil" is the most fitting term to describe the act of thwarting someone's efforts or intentions.