D. Seeing the happy times of his youth upsets Scrooge because he knows that they are gone now. Scrooge is pretty miserable in his current life. Also, A B and C don't make sense because for A he remembers being there when he was younger, so it's not something he imagined, B has nothing to do with the story really and C Scrooge was very happy in his younger days. So D is the correct answer.