"The granddaughters visit their grandparents."
Translation:La nepinoj vizitas siajn geavojn.
mi vizitas miajn geavojn
vi vizitas viajn geavojn
li vizitas SIAjn geavojn
ŝi vizitas SIAjn geavojn
ĝi vizitas SIAjn geavojn
ni vizitas niajn geavojn
vi vizitas viajn geavojn
ili vizitas SIAjn geavojn
If you used "iliajn" you would be referring to another "ili" (other people, than the "ili" the sentence starts with. That means that in your sentence "La nepinoj" would be referring to somebody else's "geavojn" not their own. (That goes for liajn, ŝiajn and ĝiajn as well)
Well, the English is ambiguous without context (even though definitely most natural is to assume it is their own grandparents, it's not necessarily so), so Fingtam has a point - "iliajn" should also be accepted for this reason.
"Do the granddaughters visit the [contextually specified group of people]?"
"No, they visit the [contextually specified group of people]'s grandparents"
ie, they (the granddaughters) visit their (the group's, NOT their own) grandparents.
Convoluted and unlikely, but perfectly valid language.
For pedagogic reasons though I could understand rejecting it even though it's technically correct, contingently, as most likely most people writing "iliajn" are doing so for the wrong reason, and could do with knowing about the difference :s
Context is always tricky in the Duolingo course because so little context is given. One thing to keep in mind, though, is that sometimes the absence of context is context. Any native English speaker would understand "the granddaughters visit their grandparents" to mean that it was their own grandparents who were visited. The theoretical possibility of a context which could reasonably change that perception does not matter if that context is not presented.
I would argue that even your example fails
- There are sailors there. Do the granddaughters visit them?
- No, the granddaughters visit their grandparents.
Even in this context I have a very strong sense that we're talking about the granddaughter's grandparents.
- Is John visiting Sam?
- No, John is visiting his grandfather.
This would be John's grandfather.
Other cases could be contrived.
- John is such good friends with Mary and Sam that he visits their grandparents. (Mary and Sam's grandparents.)
- John is very close with Mary and Sam. Today he is visiting their grandparents. (Whose grandparents? It's not clear.)
All this put more simply - an ambiguous pronoun is when more than one possible antecedent exists. In this case, it's the lack of context which makes the pronoun unambiguous, since it's only through the addition of context that additional possible antecedents could be introduced. Since there is no other context, there are no other antecedents.