In English the use of "what" and "which" differ in that: "which" is used when the options are known, whereas "what" is used when the options are not known. For example, if you and I are talking at a bar I may ask "what is your favourite car?", but if you and I are talking at a car show room, I may ask "which is your favourite car?" In Spanish that difference does not exist. In both cases the question will start with "cuál".