"You are a human."
Translation:Du bist ein Mensch.
What's wrong with using the familiar singular form to answer the question?
"Du bist ein Mensch."? It shows up as the suggested translation here, so maybe something else was wrong with your answer?
"Du bist ein Mensch" and "Sie (formal singular) sind ein Mensch" are both correct.
Sie sind ein Mensch is currently not accepted.
As you say, it's correct, so if someone wishes to use this form, please report it if it gets rejected.
As I said, that's currently not accepted even though it should be. The Pearson team who added this sentence hadn't added that as an acceptable translation.
Report it and perhaps they will add it in the future.
I don't think the Pearson editors read these sentence discussions, so there's no point in posting about it here. Just report it if that makes you feel better -- but I don't think they pay attention to the reports, either.
Wrong case: einen is masculine accusative, but predicates after “to be” are in the nominative case in German.
Also, Mensch has to be capitalised.