Duolingo doesn't do a very good job explaining it: Here, "da" is kind of a relative pronoun, referring to something that needs to have been stated before. For example, first in english: Is your mother dead? I don't speak about that. "That" refers to the fact that the mother is dead.
Now in german: Ist deine Mutter tot? Ich spreche DArüber nicht. Where "da" refers to the dead mother again, and the extra "r" is added to make the word easier to pronounce.
You can use the same construction for other prepositions as well, for example: Cathy und Sonja gehen oft ins Kino. Emily träumt DAvon. Here "da" refers to the fact that Cathy and Sonja go to the cinema, and Emily dreams about that.
Hope it was clear enough!
Thou speaketh not of the truth, or either, knowest ye not of the old way. Old English is weird to use and duolingo shouldn't even bother, but technically, voidlogic is right.
But please don't mark your answer as correct, voidlogic. Because that'll probably just mess up the system. Too many varieties allows even the lamest sentences to have a chance of passing.
IF you login from a computer, there are clickable icons under the field for answers. IF you use a mobile device, longpress on some letter should show you all localizations. IF you still don't get umlaut-powered letters, probably should install additional keyboard on your mobile device (search the internet for instructions how to do that, depending on the type/manufacturer of your device)