The correct option is
A mumbling, pitter-patter
An onomatopoeia is 'the formation of a word from the sound associated with what it's named'.
Option A: 'Mumbling' is a sound-word that imitates the sound when someone's mumbling. Similarly, 'pitter-patter' also expresses the sound of the rain.
So, these words are onomatopoeic and option A is correct.
Option B: 'Rain' is a noun that refers to the precipitation experienced by the atmosphere. It does not express a sound.
Option C: 'Barely' is an adverb that means 'only just, almost not'.
Option D: 'Pitter-patter' is an onomatopoeic word, but this option does not identify the other word 'mumbling'.
Hence, options B, C and D are wrong.