- Which is your most favourite subject in school ?
- What is your most favourite subject in school ?
Which one is acceptable? If both are acceptable, do they have any difference in meaning?
- Which is your most favourite subject in school ?
- What is your most favourite subject in school ?
Which one is acceptable? If both are acceptable, do they have any difference in meaning?
In short, when the interrogative pronoun which is used, it is asking about something among a group of things.
Note: which can also be used as a determiner.
They aren't exactly equivalent. "Which" should be used when the choice is to be made from within a defined, finite set of options, as in, "Which of these is your favourite: Math, English, or Social Studies?" "What" should be used when the answer to the question could be almost anything, and is not presupposed to come from a limited subset of all possible answers.
By the way, "most favourite" is incorrect. "Favourite" is the top (preferred) choice from within a group; there can be no comparative form of this, hence "most favourite" is simply impossible, and hence wrong. (If something is the top choice, it can't be "more" top or "most" top; it's the top choice or it isn't, and no degree of being the top is possible.)
Well, Chris Dwyer's answer link is sufficient for you since it states that
"Which" is more formal when asking a question that requires a choice between a number of items. You can use "What" if you want, though.
Generally speaking, you can replace the usage of "which" with "what" and be OK grammatically. It doesn't always work the other way around, however. There needs to be a context of choice.
You can use both which and what for your question since it has a context of choice.