This is a most strongly supported question. We are given a series of assertions regarding newborn's and the comfort they receive from their mothers that goes something like this:
1. When newborns are crying due to discomfort, merely hearing the mothers voice will lead to a temporary halt in crying.
2. Hearing the voices of others does not have this effect.
(A) "more easily learn to recognize . . ." We are not told about the comparative ease with which they learn their mother's voice vs. other people's voices. We are only told that the mother's voice brings comfort where other voices fail to do so. Unsupported. Get rid of it.
(B) ". . . is the first thing a baby learns to recognize." Once again we are only told that hearing the mother's voice brings comfort where other voices do not. We are not told that their voice is the first thing they learn to recognize. They could learn to hear daffy ducks voice first, yet daffy duck could still neglect to bring them comfort when they are experiencing discomfort. Unsupported. Get rid of it.
(C) This is the best answer. Another way to look at this question would be to rephrase the question stem to "What conclusion would most follow from the facts given?" Read the two assertions I listed up top then say "therefore" like it's the conclusion and read this answer choice. It should perfectly line up. The one thing that gave me a hard time selecting this at first was that it said "babies" instead of "newborns 4 to 6 weeks" which is what is listed in the stimulus. If you keep a sharp eye, but maintain flexibility on a question like this then you will know that these 2 terms are interchangeable. Also the rest of the answer choices really suck.
(D) This is a tricky answer choice, but if you look at the first two lines of the stimulus you will see that the stimulus limits the scope to "newborns . . . whose mothers have been the primary caregivers . . ." Whereas the answer choice says "primary caregivers" which in some newborns could or could not be the mother. Perhaps the primary caregiver in the case of some newborns is someone other then the mother, and the voice of the primary caregiver in that situation does not bring comfort but someone else's voice does. We just don't know have enough information to make that judgement. This answer choice is too broad to be supported so we can get rid of it.
(E) This was another tempting answer choice, but is ultimately not supported. We know that in cases where the mother is the primary caregiver that their voice relieves discomfort. This does not mean, however, that discomfort in newborns is best relieved by hearing the mother's voice. Perhaps there is a toy that relieves the newborns discomfort better than the mother's voice. The toy doesn't have a voice, but relieves the discomfort better than the mother's voice. We just don't know, and so this answer choice is unsupported by the information in the stimulus. Get rid of it.