The correct option is
B muttering, chanting
An onomatopoeia is 'the formation of a word from the sound associated with what it's named'.Option C: 'Muttering' is a sound-word that imitates the sound of a barely audible utterance. Similarly, 'chanting' also expresses the sound of speaking or singing.
So, these words are onomatopoeic and option C is correct.
Option A: 'Muttering' is an onomatopoeic word, but this option does not identify the other word 'chanting'.
Option B: Similarly, 'chanting' is an onomatopoeic word but this option does not identify 'muttering' as one.
Option D: There are two onomatopoeic words 'muttering' and 'chanting'.
Hence, options A, B and D are wrong.