this is the girl who won yhe quiz competion last week.
In this sentence the bold word is which pronoun
"Who" is a relative pronoun.
A Relative Pronoun points or refers to the noun going before it, which is called its antecedent.
Example: I have bought the book that I wanted.
Here, the pronoun 'that' relates to the noun going before it i.e. to 'the book'.
Other examples of relative pronouns are ' which', 'when', 'who' (whom, whose) and 'where'.Hope this helps!