The adjective "plausible" is used to describe nouns (ideas, people, things) that are seemingly valid, likely, acceptable, credible or truthful. You can also infer this definition by looking at the surrounding information of the word (also called "Context Clues"), more specifically, to the other clause "while her adversary’s argument was neither reasonable nor valid" which gives you hints that "plausible" is the opposite of "reasonable" and "valid."
