The phrase "was plausible" refers to something that seems believable or reasonable. There are several synonyms one could use to convey this same meaning, such as "credible," "convincing," "tenable," "authentic," "feasible," or "acceptable." Another alternative could be "persuasive," which implies that the argument or statement in question is able to sway or convince others of its validity. Additionally, "logical," "rational," "sound," and "reasonable" all convey a sense of coherence and coherence that supports the plausibility of a statement or argument. Each of these synonyms can be used to describe a situation, claim, or explanation that presents convincing evidence or argumentation.