Which word refers to reasoning that seems plausible but is actually flawed?

The term that accurately describes reasoning which appears plausible but is fundamentally flawed is "sophistry." This word originates from the practice of the Sophists in ancient Greece, who were known for their skillful but often deceptive reasoning. Sophistry involves arguments that may sound convincing at first but lack genuine validity or truth upon closer inspection. This concept aligns neatly with the question's requirement for a term that captures the essence of deceptive reasoning.

In contrast, the other choices do not relate to flawed reasoning. "Stentorian" describes a loud and powerful voice, while "salacious" pertains to something that is indecent or sexual in nature, often used to describe inappropriate or lascivious content. "Stygian" relates to the River Styx from Greek mythology and generally connotes darkness or gloom. Thus, these terms do not fit the context of flawed reasoning, making "sophistry" the clear and correct choice.