Answer :
Dr. Marlene Zuk, a professor who studied crickets on the Hawaiian island of Kauai, discovered that a mutation in the genes of male Kauai crickets prevents their wings from making a chirping sound.
The sentence begins with 'a professor' and the clause that follows gives us additional information about this professor, so we need a pronoun that introduces a defining relative clause. In this context, the proper choice is 'who.'
When practicing identifying the relative pronoun in English, you will notice that it often replaces a noun in the previous clause and introduces additional information about that noun.
Editing paragraphs to correct pronoun agreement errors involves ensuring that the pronouns agree in number and person with the nouns they replace. In the given paragraph about Diana, 'they' should be replaced with 'she' and 'it' with 'they' as appropriate to match the singular or plural nature of the noun and maintain the correct point of view.
Therefore the correct answer is who.
