I don't like any of the answers...
A) Babies more easily learn to recognize the voices of their mothers than the voices of other people.
Seems to be out of scope and not necessarily supported
(B) A motherâ€™s voice is the first thing a baby learns to recognize.
Definatley out of scope and not supported
(C) Babies associate the voice of the primary caregiver with release from discomfort.
I vote for this one, this association is demonstrated by the empirical evidence.
(D) Often only a primary caregiver can provide comfort to a newborn.
Close to being supported, but a bit extreme.
(E) Discomfort in newborns is best relieved by hearing the motherâ€™s voice.
Definatley not supported.
I choose C.