A：The first issue with (A) is idiomatic: we have "not...nor" instead of "neither...nor." But it's best to be conservative when it comes to idioms, so let's chalk this up as a strike against (A) and move on.

Next, we have two descriptors for the children, so let’s see if the sentence works for each one individually:

1) children {...} were not preoccupied ABOUT the reasons {...}
2) [children] {...} were not even very curious ABOUT the reasons {...}

You can be curious ABOUT something, so the second bit works here. But you can only be preoccupied WITH something, not preoccupied ABOUT something. So we have another idiomatic issue and our second strike against (A).

The final issue (which does not involve idioms) is that the second part of the sentence is a bit redundant—we don’t need to say that they were reasons "that led to" their divorce because that’s what reasons do... something like “reasons for their divorce” would suffice. That gives us our third strike against (A).

If you aren't sure about the idioms, you'll have a tough time eliminating (A) right away. But with three votes against it, (A) isn't looking good.