Which interrogative pronoun should replace the blank space in the following sentence?: _____ shirt do you think looks better on me?
There are five interrogative pronouns in English: which, what, who, whose, and whom.
In this case, the rest of the sentence seems to assume that the speaker has two shirts laid out and is asking someone to pick the one that looks better.
When selecting from a group of items, there is only one choice: which.