this is the girl who won yhe quiz competion last week.           
In this sentence the bold word is which pronoun

Dear student
"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!
Regards