The word that best matches the italicized the word 'serene' would be 'peaceful' as it gives the same meaning. <u>'Serene' stands for 'something calm, soothing, unruffled, and without any kind of worries, anxieties or disturbance' and the word 'peaceful' conveys the similar essence of being 'motionless and calm' which the speaker feels during 'autumn in countryside.</u>' Thus, <u>option C</u> is the correct answer.
