Use what for general items and which for specific things
Reader’s question: Which of the following is correct and why?
What information do you require?
Which information do you require?
Answer: In the examples you gave, what is open-ended and general and which is more closed and specific, i.e. asking for a choice between a specific, limited number of items. For example:
What information do you require? – I want to know how to register, which form to use etc.
Which information do you require? – I want the information for non-Australian residents, not the information for Australian residents.
In practice though, we often use these words interchangeably.
What is your favourite book?
Which is your favourite book?
Some people regard which as more formal.
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