If you get a D+ in one undergraduate class, does that mean you need to retake it in your masters?

You don't "have" to retake it, but it does bring your GPA down.it is still a passing grade for graduation (depending on the university but at most it's still considered passing). You would need to ask admissions at the university you are attending and they can refer you to someone who would know best-it also depends if you're at a university that deletes a lower grade class and replaces it with the new grade or if they just average them  together.