I think "this is a wrong building" is also correct. "The wrong" would mean there are only two buildings: one correct and one wrong. If there are more buildings than two, then there is one correct and a few wrong ones. So "a wrong" should also be treated as a correct answer.
same here with the "oikein, tämä on oikein stadion". väärä or oikein are both correct in my opinion.
If someone asks: "is this the wrong building?" i could say: "wrong, this is the correct/right building"
Edit: understood my mistake.... The thing is: check the bending of the word väärä vs väärin Oikea vs oikein
OK I get that väärä and oikea usually come before nouns and they indicate that something is the the right or wrong thing, but then why do you say 'Onko tuo vastaus oikein/väärin?'
People are saying that oikein and väärin stand alone and are adverbs, but in English in the sentence I just gave they would be adjectives. I'm not arguing with you Finns, I'm just a bit confused. Thanks guys!