If its not required then its fine but I'm sure the D is still calculated in your overall GPA. I had to retake astronomy (I failed it awhile ago) cuz the F brought down my gpa. It really helps to retake anything below a C. I retook anything below a B lol
I'm not sure why you're taking a class such a statistics if it isn't a pre-req for your program, but in any case you took it and now have a D in it. My advice is to re-take it and aim for a better grade, otherwise that D will stay on your transcript and bring down your cumulative GPA quite a bit. Also, many nursing programs require statistics, so if you do not get accepted in the program you apply for, and attempt to apply somewhere that does require statistics, you'll be required to re-take it anyways for a much better grade. Better to re-take it now and get it out of the way.