We drove past my old school, which is celebrating its 100th anniversary this year. Here, ‘which’ is a ________ pronoun.

Relative pronouns introduce relative clauses. The most common relative pronouns are who, whom, whose, which, that.
The relative pronoun we use depends on what we are referring to and the type of relative clause.
- We don’t know the person who donated this money.
- We drove past my old school, which is celebrating its 100th anniversary this year.
- He went to the school (that) my father went to.

যে pronoun পূর্বে উল্লেখিত কোন Noun বা Pronoun কে নির্দেশ পূর্বক দুটি বাক্যকে যুক্ত করে তাকে relative Pronoun বলে।
- Who, which, that, what, but, as - এগুলো relative Pronoun রূপে ব্যবহৃত হয়।

Source: A Passage to the English Language, S M Zakir Hussain, Cambridge dictionary