Which word signifies something that seems plausible but is actually fallacious?

The word that signifies something that seems plausible but is actually fallacious is "specious." This term is often used to describe arguments or beliefs that have a deceptive attractiveness or appeal, yet lack genuine validity or truth. When something is specious, it may look accurate or convincing on the surface, leading one to initially accept it, but upon deeper examination, it reveals its inherent flaws or misleading nature.

In contrast, "valid" refers to arguments or assertions that are logically sound and backed by reliable evidence. "Authentic" denotes something genuine and real, and "legitimate" implies compliance with recognized standards or laws. While these words convey a sense of correctness or truth, they do not capture the essence of something that falsely presents itself as valid or true, which is why "specious" is the most accurate choice in this context.