What kind of pronoun is used to form questions?

The interrogative pronouns introduce questions.

They are: who, whom, what, which, whose.

Examples:

What time does the movie start?

Who ate the last cupcake?

Which computer did you buy?

Whose mailbox did you hit?

To whom are you sending invitations?

Note: The interrogative pronouns also function as relative pronouns which introduce a relative clause.

Example: The man whose mailbox I hit was very nice about it.