(a) The word 'Blithesome' means ' light hearted , merry , cheerful' which is nearest opposite in meaning to 'sullen' which means ' gloomy or dismal' , morose' which is option (b), theefore (b) is the correct answer . The other 3 answer choices have following meanings: graceful-elegance of form, adventurous - hazardous , Mammoth-immensely large.
