"Is this water yours?"
Translation:Эта вода ваша?
Yes. You have to say "Эта вода ваша?" (Is this water yours?) or "Это ваша вода?" (Is this your water?). For this task, you can only say "Эта вода ваша?" or "Эта вода твоя?"
It's grammatically incorrect. Эта here is modifying вода, "this water". Does эта вода belong to you? Эта вода ваша?
Это = It (is your water).
Эта (feminine)/Этот (masculine)/Эти (plural)/Это (neutral) = This (water of yours). It is a demonstrative.
So in a very similar exercise "Is this cat yours?" was "Кошка ваша?" No need fir этот, herr i got it wrong for omitting эта. Is this an animate/inanimate thing or something else?
This is silly. I understand you want to emphasize one specific construction, but there is no lexical difference between "Эта вода ваша?" and "Это ваша вода?" -- so marking one correct and one incorrect is sort of dickish.
If I understand correctly, because it's specifically asking about THIS water, you must conjugate эта to be feminine and immediately preceding the noun. In your example, I can see you understand this but I think it would be more inconsistent for the system to mark your alternative correct.