I think that is too general. They're asking for a specific mother (and, also, isn't that gramatically incorrect in english? Genuine question, since it isn't my first language)
Without an article, it should be capitalized since it is serving as a name or maybe title. But you definitely can say "Who is Mother?" without the article.
The audio sounded like "Quis est pater" to me so I entered that and it marked it as correct, yet it is saying that the translation is "Who is the mother" when pater means father? This is a discrepancy. Either the audio or the answer ought to be changed.