Originally, "mor" is a shortened form of "moder", of which the plural is "mödrar". The plural is still the same, but "moder" is rarely used at all anymore.
Not that there are many situations where this might happen, but would that come out when pluralizing mormor or farmor as well? For instance, my maternal grandmother and your maternal grandmother would be våra mormödrar? Or if my mother had two mothers mina mormödrar?
Why is mödrarna not the plural form of mothers? Maybe I'm missing something but I thought that would have been plural while mödrar would have been singular.
I could be wrong but it's asking for Our Mothers. Mödrarna would be The Mothers.
Is mommarna a word? I thought I had seen that earlier in this lesson, but it didn't work.