PT147 · S4 · Q14 Psychology researchers observed

We already knew that parents feel emotion when singing to their infants. This answer lets us know that they feel more emotion if its their children vs. other children. (Obvi, other people's children are gross) The fact that parents feel more emotion around their kids than around other kids makes it seem like the level of emotion around their kids is potentially the highest level of emotion. That weakly strengthens the author's storyline, although we still don't have any sense of whether feeling emotion actually changes the sound of the singing.

It doesn't really matter whether the parents knew they were being recorded or not. We wouldn't want to let them in on the aim of the study, so we would hope they don't realize that what's being listened for is "infant-present" vs. "infant-absent" emotion. Either way, this answer makes us feel somewhat better about the "blind" aspect of some of the participants who didn't even realize they were being recorded (because a more 'honest' performance probably came out), but that's a very weak strengthener.

It feels like (A) and (C) would accomplish similar levels of Strengthening and thus kind of cancel each other out as viable answers. Since the author is trying to sell us on the idea that the emotion parents feel while singing to their kids has a noticeable effect on their voice, he is definitely assuming that "there's a difference, both emotional and sonic, when it comes to parents singing to their kids vs. not". So (C) provides a dissimilarity the author is assuming (they feel more emotion when it's their kid vs. nothing), and (A) provides a dissimilarity the author is assuming (they feel more emotion when it's their kid vs. some random kid). But we're not especially worried about the notion of whether parents do / don't feel a ton of emotion when they're around their infant. We're especially worried about whether that emotion changes the sound of their singing voice, and whether that's how 80% of the time the psychologists could guess right.

We might say this weakly strengthens, since people might be decent judges of whether emotion is affecting their singing. But to LSAC, it's opinion in (E) vs. involuntary physiological response in (D). Science > Opinion (at least it still works that way in LSAT's wonderfully old-fashioned universe). The opinion we'd be even more interested in hearing would be the psychologists, who were the ones that were actually judging the sound. If most of them told us "we noticed that the singing sounded different when infants were there", then that would strengthen the author's story and push back against alternate versions of how they were able to guess right 80% of the time.