which
The Italian word 'quale' is used similar to the English word 'which', for asking questions or specifying one or more people or things from a definite set. It generally precedes the noun it refers to. Keep in mind, the word may change forms depending on the gender and quantity of the object referred to.
Which book do you prefer?
This sentence is using 'quale' to ask a question about a preference. 'Quale libro preferisci?' means 'Which book do you prefer?' in English. Here, 'quale' is used in the singular because it's referring to a singular object (book).
I don't know which one to choose.
This is another example of how 'quale' can be used to refer to a selection, choice, or preference, but without specifying the object. 'Non so quale scegliere' translates to 'I don't know which one to choose' in English. 'Quale' is used here in a general sense.
Which road should I take to get to the airport?
This is an example of using 'quale' to ask for directions. The sentence 'Quale strada devo prendere per arrivare all'aeroporto?' translates to 'Which road should I take to get to the airport?' in English. It shows the use of 'quale' for asking about a choice among pointed out options.