Antonyms are words with opposite meanings. Options A, B, C, and E are all true antonym pairs: impassive (emotionless) vs ardent (passionate), phlegmatic (calm) vs zealous (fervent), vehement (furious) vs listless (lethargic), and fervid (intense) vs apathetic (uninterested). However, 'lukewarm' and 'indifferent' are NOT opposites - both suggest a lack of enthusiasm or passion. They're similar in meaning, making this the non-antonymous pair.