Making up the context for me is half the fun of Duolingo. I hope they never stop with the quirky sentences.
Really, the best would be "our" as Latin often implies an obvious possessive. Context would obviously change that, but I think the most natural would be "We save our mother."
Servare is conjugated as follows for the present tense active indicative: servo, servas, servat, servamus, servatis, servant. Servamus is the first person plural, which means it translates not to "save mother" (an imperative) but to "We save mother." Latin is a very word-poor language, and most verbs have a range of nuance and meaning. Sevare can also be translated as to watch over; protect, store, keep, guard, preserve, or any other meaning along those lines. Hope this helps!
It's a common feature of most ancient languages, not just Latin. Human beings had a habit of making up more words over time, it seems. And it's both a blessing and a curse when learning ancient languages. There's less vocabulary to learn, to be sure, but it makes sentence more ambiguous and easier to mistranslate, especially without context.
Latin nouns change their ending depending on what they do in the sentence. Mater is a subject (nominative) and matrem is a direct object here (accusative).