Which interrogative pronoun should replace the blank space in the following sentence?: ___ sweater is this?
Whose is related to possession (whose book, whose sweater, whose victory). The question is asking whose sweater it is/who does this sweater belong to.
Who and whom refer to people (who is there, who are you, etc.). What usually refers to a thing (what book is that, what game are you playing, etc.). Which is used to ask about a specific thing out of many options (which book is that, which classes are you taking next year, etc.).