"Which man? - This man!"
Translation:Welcher Mann? - Dieser Mann!
I wrote Welchen Mann? Diesen Mann! I know its wrong now, but why is it wrong? Is it the case?
Yes, it is the case. You used the accusative case, when the nominative case is needed (there was no indication in the sentence that any other case would be correct)
I would say there is no hint why you have to use nominative here, so also accusative should be accepted. It could be meant like "I've seen the man." "Which man?" "This man!" - Ich habe den Mann gesehen. Welchen Mann? Diesen Mann!
What the heck is with the hyphen in the English translation? Please pay attention to correct punctuation, Duo!
Actually, putting everything else on a 'back burner' for now — it's a dash that DL should have used for their intended treatment (right or wrong) for this sentence.