GMAT Sentence Correction (SC) | Re: A 2009 study from the California State Housing

Hi experts,
Hi why option D is preferred?
Even though we have two items in the list,it is not required to put the comma between this two items but it's not this case.For P and Q and R,we couldn't know Q is part of which group. Isn't it better to clearer if the comma is inserted?

Thanks