Interrogative pronoun :

In traditional English grammar, the term interrogative pronoun refers to a pronoun that introduces a question. These words are also called a pronominal interrogative. Related terms include interrogative, "wh"-word, and question word, although these terms are usually not defined in precisely the same way.

In English, who, whom, whose, which, and what commonly function as interrogative pronouns. When immediately followed by a noun, whose, which, and what function as determiners or interrogative adjectives. When they start a question, interrogative pronouns have no antecedent, because what they refer to is precisely what the question is trying to find out.

Examples:

Interrogative pronouns are all around us, whether you knew the name of them or not as you speak and read. Here are a a few examples from literature and other sources:

"Even if you do learn to speak correct English, whom are you going to speak it to?

"(attributed to Clarence Darrow)

"When a man tells you that he got rich through hard work, ask him: 'Whose?"

(Don Marquis)

“I have water and Diet Coke. That was the only soft drink I allowed Howie to have. Which do you prefer" (Stephen King, "Under the Dome." Scribner, 2009)

"'What did you see down there in the kitchen?' Caddy whispered. 'What tried to get in?'

"(William Faulkner, "That Evening Sun Go Down." "The American Mercury," 1931)

"I got a belt on that's holding up my pants, and the pants have belt loops that hold up the belt. What is going on here? Who is the real hero?"

(Comedian Mitch Hedberg)

Semantic Contrasts: What Versus Which:

Whether you use what or which in a question depends on the context of the question, whether there are specific items to choose from (which), or whether the question is completely open-ended (what). Of course, casual conversation brings exceptions.

"These pronouns express two semantic contrasts:

"(1) a gender contrast of personal (the who series) and nonpersonal (what, which):

Who is in the woodshed? What is in the woodshed?

(2) a contrast of definiteness: indefinite what contrasts with definite which—the latter always implying a choice made from a limited number of alternatives:

What was the winning number? [you must recall what it was]

Which was the winning number? [you have a list of choices]

"Note also the use of what to ask about a role or status:

What is her father? [a politician]

Which is her father? [in the photograph]"

(David Crystal, Making Sense of Grammar. Longman, 2004)

"What is used when specific information is requested from a general or open-ended possible range. Which is used when specific information is requested from a restricted range of possibilities:

"A. I've got your address. What's your phone number?

B. Oh it's 267358.

(an open-ended range of possible information)

[looking at a pile of coats]

A. Which is your coat?

B. That black one.

"However, where the number of options is shared knowledge among speakers and listeners, what + noun is often used in informal contexts. Here, what is an interrogative pronoun used as a determiner:

"[talking about a shop]

What side of the street is it on, left or right?

(or: Which side of the street is it on?)

"A: Did you see that documentary about the SARS virus last night?

B; No, what channel was it on?

(or: Which channel was it on?)"

(R. Carter and M. McCarthy, "Cambridge Grammar of English: A Comprehensive Guide." Cambridge University Press, 2006)