Nasza matka -> Nasze matki -> Marked correct Nasza mama -> Nasze mamy -> Marked incorrect
Aren't both correct, and isn't "nasze mamy" the more colloquial way to say it?
There is another question on this unit asking us simply to translate 'mother' - and the answer is 'mama' there
The thing is, that in English 'mother' is of course more formal than 'mom', but still quite a normal, neutral word. In contemporary Polish, "matka" is quite formal, it's not neutral, so we would prefer that only "mother" was accepted here.
However, it turned out recently, that the algorithm accepts "mom" every time when "mother" is accepted (and the other way round as well, I think), even if we don't put it among accepted answers.
"matka" is currently put as a 'version' of the word "mama", so I guess we shouldn't disregard any of the translations in the exercise where you just translate "mother"...