I'm a bit confused with this sentence, it seems sometimes I can leave out the article but here I need to include the article 'O' for it to be accepted. Is 'Qual é meu quarto' incorrect?
I'm not sure if it is grammaticaly incorrect to say "qual é meu quarto" but it doesn't sound right. You are asking for a specific room and the "artigo definido" is desireable. I know the "meu" is already kind of specific but Portuguese is sometimes redundant.