We ask What...? when there are a lot of possible different answers. Example: What sport do you play? (There are many sports.)
We ask Which...? when the answers are quite limited. Example: Which way should I turn? (You can turn right or left only.)
If we include the possible answers as part of the question, then we use Which. Example: Which restaurant did you go to last night? 'The Hot Potatoe' or 'Pine'? (There may be many restaurants but in the question we are giving only two choices.)
If we don't include the options, the answer may be any restaurant, and that's why it should begin with What. Example: What restaurant did you go to last night?
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