How are these terms related? plausible : believable synonym antonym neither a synonym/antonym, but related unrelated skip?

I think the correct answer would be the first option. The terms plausible and believable are synonyms. Plausible means something that is probable, acceptable, it is likely true. Believable would mean the same as well, it is something that is realistic, credible or acceptable. Words or phrases that means the same are synonyms.