"Qual è la macchina più costosa?"

[SaxyLady]

Often I've seen "qual/quale" mean "what" but here when I used it that way I got it wrong (correct answer had used "which").

[Lodoviko]

"What" is more colloquial in English, but attracts no negative attention. "Which" is more correct and should be used in writing. Otherwise, they are interchangeable in any real practical sense.