Your best to email universities admissions departments. But, usually if it states a C then it usually unlikely that you may be considered with a D because their is often a lot of competition.
A course I wanted to apply for required a B at Nat 5 Maths. I have a C and they said that I wouldn't likely be considered unless I resat it.
I would highly recommend resitting National 5 Maths. There are a lot of resources for this now which makes it a lot easier.