This is the 482nd most frequent Dutch word.
welke
"Which."
'Welke' is used here as a question word to ask for a specific choice from a range of options.
Welke film wil je vanavond kijken?
Which movie do you want to watch tonight?
'Welke' introduces a relative clause referring to an unspecified choice among books.
Ik weet niet welke van de boeken het interessantst is.
I don't know which of the books is the most interesting.
'Welke' is used as a pronoun to refer to an unknown or unspecified item mentioned earlier.