Correct Answer:
A. Unlikely
The word 'plausible' means seeming reasonable or probable. To find its antonym, we need a word that means the opposite. Unlikely (Option A) means not probable or not expected to happen, making it the direct antonym of plausible. Feasible (Option B) is a synonym or similar in meaning to plausible, as it means possible to do easily or conveniently. Fantastic (Option C) means extravagantly fanciful, unrealistic, or excellent, which is not a direct opposite of plausible in terms of probability. Thus, 'unlikely' is the best antonym.