GMAT Sentence Correction (SC) | Re: Visitors to the Rodin Museum have often examined the

Isn't the usage of "their" in option D a spot of concern?

Also, can somebody point me to some good articles on "they" and "them".

Because all I know about "they" and "them" is that they are gender-neutral and strictly plural pronouns. But, it looks like I need to understand them more carefully, especially their usage in long sentences.

---