Any time you see a "missing" correct answer, you should report it. That way, the database gets better every day. I think that is one way that we "early adopters" can "repay" Duolingo for the free use of their resources.
someone suggested we should report additional correct sentences, but I said you can prove this with them (which as a math teacher, I might well say--at least in English) and I have no idea if that is really wrong or just marked wrong.
Comprabante = Proof. It should say, "I can prove this with them".
I chose "I can explain this with them." Closest verb to "prove" I saw the option for...maybe "verificar" should be in the sentence instead.