Recently I've come across sentences that doesn't have "one" in it and it looks like odd to me because I'm used to say "which one...?" The sentences must be correct because they are from a grammar.
Examples:
Which do you prefer - meat or fish?
You can have tea or coffee - which do you prefer?
To me,"which one" would be more correct. I don't know. Perhaps sometimes "which" and "which one" can be interchangeable.
The "one" could imply that of the alternates only ONE choice is possible, or permitted. "Which" alone could indicate several choices from the set of alterates could be selected in various combinations. Of course, speakers are often very imprecise about their meanings & intentions when saying "which" or "which one". Usually the context makes it clear if the choices are mutually exclusive or not. If it's obvious the choices exclude each other, than a speaker would say "Which" with the listener understanding there's a missing "Which (one)".
It's not normally a problem unless the speaker does say "Which one do you want", without emphasising "Which ONE do you want?", when the listener would like to make multiple choices. They'd have to recognize the mis-match & communicate more about what is possible and what is not.
English is not my mother tongue, thus I can be pretty wrong with my opinion, but with "which" I understand an option among some different objects. "Which do you want [to eat], cake or pie?" With "which one" I understand a selection among several things of the same kind. There are many books on the teacher's desk, a student asks the teacher to hand out him his book, but the teacher doesn't know what book belongs to the student and he asks him "which one [is your book]?".
