What's the purpose of the "Would you kill a baby if it would cure cancer" question?

I would need to rewatch it again for the full context, but I think the question presents a couple of things for the viewer to take away:

It at first is about Nora's and The Man's morality which is being questioned/contested and not necessarily because either did or didn't take a pro-choice and/or quality of life or 'the many over the few' approach to the problem, but most likley because neither didn't ask a lot of questions or hesitate in finding other solutions, or showed universal empathy.

And While Nora did ask questions about the nature of the Child's end, she also was strongly compelled by her bias when she asked if they were her twins and made a point of her own personal needs, but then once they said they were not hers, she was extremely dismissive, saying, "Kids die every day. What's one more." She ultimately showed a lack of empathy for children who are not hers...

A simplified interpretation then might be that it was a question about "playing God". That perhaps in the preview of the scientists, they felt that candidates that are more honest in either "not knowing" what the solution to problem like that is or rejecting the solution outright, as the only possible solution, and/or showing empathy and consideration for all life makes sure that they wouldn't affect the other reality with preconceived notions of their own belief system or bias, as the the twins are probably a metaphor for the two realities or parallel universes. It was an ethical question to determine how one might react to what they find on the other side.

This also goes hand in hand with the season finale too, in terms of plots reflectively turning into metaphors,

as we don't really know what the truth is of Nora's experiences, nor does it matter, because Nora has always been this walking contradiction that just needed to be to accepted, because of what she feels, not necessarily because of what anyone knows or doesn't know of what she's done. And it's that faith in the unseen that could also be translated into a type of innate love that much humanity has been built of off.

Here's what Leftovers co-creator Damon Lindelof said in an interview with Alan Sepinwall, published shortly after the finale aired:

[Sepinwall] What answer did the scientists want about killing the baby? The guy in the VW who burns himself alive gives one answer. Nora gives the other. Both are rejected.

[Lindelof] I think that the question of “What did the scientists want?” is not the operative question. Here are two other more interesting questions to ask, potentially. Question number one is, what are they measuring when they ask this question, and as a codicil to that, is the actual verbal response relevant to whatever it is they’re measuring? I would just rephrase it that way. I’ll say, what they are measuring is attachment. Both of them gave answers that suggested to the questioners that they were still attached.

[Sepinwall] You finished shooting the season in September…

[Lindelof] You’re not going to ask me what Mark Linn-Baker said when he was asked the twin baby question? That pinata is just hanging there.

[Sepinwall] Fine. What did Mark Linn-Baker say when they asked him?

[Lindelof] I’m not telling.

[Sepinwall] (Expletive deleted)

So it seems that the co-creators are maintaining a level of studied ambiguity on what the "right" answer would have been. However, Lindelof indicates that the scientists were not just evaluating this answer on its content, but also on the emotional and psychological affect displayed in them.

Beyond that, it may remain a mystery; and as all we know from this show, we may have to let the mystery be.

The "acceptable" answer is "my moral judgment, to a hypothetical question, has nothing to do with my ability to decide on where i want to be/go" there are quite a few ways to decline to answer and to elaborate - like "how do I know whether other baby has qualities greater then curing cancer?" or just "just how do i know which future line to pick?" ... either way, at least according to the screenplay, one needs to show that they know something either about moral, or about physics, or about "relativity" of both - asking "are the twins mine?" is so irrelevant (as Bekker puts it) that it shows an average or (for the purpose) too low IQ