Identify the relative pronoun : sondra was the only person who brought flowers

I think it might be " who"

Well, there is only one pronoun in the sentence, and that is the word WHO. So the correct answer is who.

The pronoun who can be either interrogative (when you are asking a question) and relative. This is the case when it is relative, which means that it connects the dependent clause (brought flowers) with the subject of the sentence.