Which feature distinguishes the phonemes /m/ and /b/?

The distinguishing feature between the phonemes /m/ and /b/ is voicing. Voicing refers to whether the vocal cords vibrate during the production of a consonant sound. In the case of /m/, it is a voiced consonant, meaning that the vocal cords are vibrating while producing this sound. On the other hand, /b/ is also a voiced consonant; however, it is articulated at a different place in the vocal tract, classified as a bilabial stop, whereas /m/ is a bilabial nasal.

The crucial distinction here isn't merely about voicing itself, since both phonemes are voiced, but rather about the manner and place of articulation. In contexts where voicing differentiates phonemes, the positions of articulation and manner play significant roles, making voicing essential for understanding their production.

Other features like articulatory undershoot, anticipatory coarticulation, and roundedness do not differentiate these two phonemes in a straightforward way. Understanding voicing and how it interacts with the place and manner of articulation is crucial in phonetics and helps to accurately classify and distinguish between different sounds in speech.