"Which is your car?"
Translation:Quelle est votre voiture ?
When you hover over a word, you get a set of possible translation of the word in various contexts and construction, not the exact translation valid for the sentence you are working on.
"which" has a variety of translations in French, of which "celui que", but not in this sentence.
You have to understand that a given comment thread can apply to more than one example. Eg: someone might come to this page from an example that requires English to French, French to English or a multiple choice. As a result sometimes it can be hard to follow what commenters are driving at.
Are you saying, in the example at the top of this page, you hovered over the English word which in Duo and got celui que as a possible translation of which but did not get laquelle or some variation?