"Which is it?"
I tried with "qual'è" and it was correct. And Duolingo tells me is "qual è" (without the apostrophe) is also right. And I just read somewhere that if you use the stressed "è" you shouldn't use the apostrophe, and that that's a very common mistake that a lot of people still make. Can some native Italian tell me if that's true, or if they mean exactly the same, or if one's more informal than the other? (Please excuse my poor English...)
I'm sorry but Duolingo is wrong. I'm a native Italian and a teacher: the only possible form is "qual è". This form is not elision (like "I'm" in English) but it is apocope. A lot of Italian people write "qual'è" but this is a common mistake, as you have said.
Could 'Qual e' also be translated as 'which one is it?' Or can it only be 'which is it?'
"quali" and "quale" I think are for masculine and feminine nouns. "quale" is used for both masculine and feminine singular, while ( I think) "quali" is used for masculine and feminine plural. In this translation you wouldn't use 'quali" or "quale" because the word that follows is a verb and not a noun. So the translation shortens to "qual e"
Which is in italian quali. However answer is qual. Why? Why didn't we write "quali è"?