Name the part of speech of the underlined word in the following clause. which is why I was agreeably surprised.

In the clause 'which is why I was agreeably surprised,' the word 'which' is a relative pronoun referring back to the preceding idea (about gifts gathering dust). Relative pronouns introduce relative clauses and refer to antecedents. It is not a preposition (no relationship between nouns), adjective (not describing a noun), or adverb (not modifying a verb). It functions as a pronoun.