Which interrogative pronoun should replace the blank space in the following sentence?: ___ would you prefer, coffee or tea?
"Which" should be used.
"What" should be used when there is an unknown or uncountable amount of objects being referred to.
"Which" is used when choosing between a limited number of items.
In this example, there are only two options: coffee or tea. Thus, "which" is used.
We can also use the way you phrased this very question to answer this question: you said, "Which interrogative pronoun..."
Since there are a limited amount of interrogative pronouns to choose from, "which" is used.